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LPV of the Month

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NOVEMBER

R Cancri

R Cnc is a bright Mira that has been well observed by the AAVSO since 1850.  Over 21,000 observations are in the AID from over 1200 different observers as can be seen in the light curve shown below which displays all observations in the AID.  It is one of the Legacy LPV Program stars. 

The light curve based on the last 2,000 days is shown below.

light curve based on the last 6000 days is shown below.

It's spectral class is M6e to M9e according to the VSX and is classified as a Mira.  The GCVS lists R Cnc as having a visual magnitude range of 6.1 to 12.3 with a mean period of 361.6 days. 

Analysis of all AID data in the period 1850 to the present day indicates an overall mean period of 362.0 days days.  There is also another signal present at 367 days that appears to be an annual alias of the main period.  Others are present at 182 and 121 days.  These could also be annual alias periods, however these are also very close to harmonics of the main period and so it’s difficult to determine whether these are alias or harmonic periods.  The light curve shape (as shown in the phase plot below) does not appear to deviate too much from a sinusoidal shape, therefore adding weight to these additional periods likely being alias periods.

A reasonable amount of CCD photometry has been reported to the AID since 2008.  B-V has been measured around 1.5 at maximum to around 2.0 at minimum.  V-R has been measured around 2.2 at maximum to around 2.7 at minimum.

The most interesting feature of the light curve, which you can start to see in the 6,000 day curve above is the marked change in the mean magnitude of R Cnc.  John Percy pointed this out initially.  This is readily apparent in the plot below where the blue line represents the mean magnitude of R Cnc.  It can be seen that a decline set in around the early 1960’s when the mean magnitude was approximately 8.0.  The decline appears steeper commencing in the early 1990’s until the present day where the mean magnitude is now around 10.5.  There is some evidence to suggest the decline plateaued at it’s current level starting around 2008.  Therefore there has been a 2.0 magnitude drop in the mean magnitude in the very short time scale of 50 years.  At the same time there has not been a significant change in the amplitude that is shown in the plot below.  The semi amplitude has only varied 1.7 and 2.0 since the 1960’s.

However as Stan Walker from Variable Stars South has correctly pointed out, you need to be very careful with the assessment of the light curve of R Cnc.  Its period of 362 days is very close to a calendar year which means that we observe the same portion of the light curve between times of solar conjunction for many years at a time.  This therefore concentrates the observations to specific magnitude ranges and this can affect the mean magnitude calculations.  For example, the peak brightness of R Cnc has not been observed since 2004.  Since that time, we have not observed the peak brightness as the star has been in conjunction with the Sun at that time.  2004 clearly had observations either side of the maximum and so we're confident the peak was caught.  This also means that the majority of observations in the last decade or so have been when the star is in the magnitide range of 10 to 12 which skews the mean magnitude calculated (as opposed to the mean brightness) to the fainter limits.  This can be seen in the figure above which shows observations for the last 6,000 days.

If we examine the light curve for 6,000 days centred around 1952, the reverse is true!  In that period, for over a decade it was the peak brightness that was well observed and most observations were made when the star was in the range of 6.0 to 8.0, thus skewing the calculated mean magnitude in this period towards the brighter side.  So great caution needs to be exercised when analysing LPV light curves such as R Cnc.  Thanks to Stan for pointing this out!

 

 

 

If R Cnc, does indeed have a variable mean magnitude, then it is noted that stars with a variable mean magnitude are rare.  There are a very few associated with Miras which are changing period rapidly, and may be undergoing a thermal pulse.  R Cnc appears to have a stable period.  An analysis has been performed to ascertain any changes in mean period in R Cnc over the past 167 years based on observations in the AAVSO AID.  The figure below suggests that there has been a "meandering" of the mean period between the range of 353 to 372 days but nothing out of the ordinary. 

A number of researchers have reported the presence of circumstellar dust surrounding AGB stars and this specifically includes R Cnc.  Karovicova (2013) observed R Cnc via spatially and spectrally resolved mid-infrared multi-epoch interferometric observations to investigate the dust formation process in the extended atmosphere.  They found that the results were consistent with Al2O3 grains condensing close to the stellar surface at about 2 stellar radii, co-located with the extended atmosphere and SiO maser emission, and warm silicate grains at larger distances of about 4–5 stellar radii.  It is possible that the decline in mean magnitude could be attributed to this dust cloud, however these researchers did not see evidence of intra-cycle and cycle-to-cycle variability or of asymmetries within the error-bars and within the limits of the baseline and phase coverage.  So maybe the observing artefact mentioned above may be a suitable explanation.

Wittkowski (2016) conducted near-infrared K-band spectro-interferometric snapshot observations of R Cnc.  Their data confirm the presence of spatially extended molecular atmospheres located above the continuum radii with large-scale inhomogeneities or clumps.  The detailed structure of the inhomogeneities or clumps show a variability on time scales of 3 months and above.  Whether these “clumps” have some influence on the mean magnitude is debatable as variability in these clumps appears to occur on a timescale much shorter than then overall brightness decrease.

Percy (1990) published a short list of Miras with variable periods, amplitudes, or mean magnitudes.  There were only a handful of the latter, and R Cnc wasn't on the list.

R Cnc is an interesting LPV to follow so all are encouraged to maintain the coverage that has been in place over the last 167 years.

References

"New insights into the dust formation of oxygen-rich AGB stars", I. Karovicova, et al, A&A 560, A75 (2013)

“Near-infrared spectro-interferometry of Mira variables and comparisons to 1D dynamic model atmospheres and 3D convection simulations”, M. Wittkowski, et al, A&A 587, A12 (2016)

“Long-term changes in Mira variables”, J. Percy, et al, ASP Conference Series 11, pg 446 (1990)


 

OCTOBER 2017

R Aquilae

Summary

R Aquilae (R Aql) is a “special” Long Period Variable (LPV) of type M (Mira) that has a long observing history with the first observations reported in the AAVSO International Database (AID) on August 2, 1856.  In total there are about 55,600 observation in AID 99% of them Visual observations.

R Aql is at 19h 06m 22.25s   +08d 13m 48.0s  (286.59271 +8.23000) making it a star that can be observed from much of the Northern and Southern Hemisphere. 

The spectrum for R Aql is M5e-M9e which makes the star an M type spectral class star.  Data from observations for R Aql in the AID, B-V values for R Aql range from about 1.6 (near Maxima) to 2.2 (near Minima) which makes R Aql a fairly red star.

R Aql is one of 8 or so LPVs that have had a pronounce change in it’s period.

The magnitude range for this star in V band is 5.5 - 12 V which makes it a good target for both visual and CCD observers.

This lightcurve (LC) graph shows all of the observations for R Aql that are in the AAVSO International Database (AID).  It shows that the Maximas and Minimas for R Aql stay pretty steady around the 5.5 to 12 V magnitude range unlike many LPVs who vary more.  There are gaps in the data early in the LC but by around 1910 until today there has been steady stream of observations reported to the AID so there is good coverage and a rich history of observations for this star.

Period Change

The above is a period graph for R Aql  is from Thomas Karlsson's monumental work of the O/C, Period and Mean Lightcurves (LC) of around 490 Mira variable stars.  The main webpage of his study can be found at http://var.astronet.se/mirainfooc.php.  

The above chart shows R Aql’s period was about 353 days in 1856 and has steadily decreased to about 270 days by @2007 but it has increased by a few days since.   This graph shows periods of times where the period for R Aql “stands still” for @10 years and it’s period might actually increase for a few years.  The star may be in one of those  “increase for a few years” periods at the present time.  More observations are needed in the future to follow the period changes for this star.  This type of change in cycle periods is very unusual for almost all Mira LPVs.

Different class of LPV star

The first handful of periods for R AQL in the AID show that the star’s period was about 351 to 354 days for least 3 cycles . 

At the beginning of the light curve in 1856 until the 1870s it appeared that the period for R Aql may have been the same for up to 15 years.  Because of the other “standstills” this star presents at times, it’s more likely that the decrease in R Aql’s period began before the first records for the star were recorded.  It’s felt by some that the period decrease may have been going on for hundreds of years earlier.

The period change of R Aql puts it into a class of LPVs that steady or sometimes quickly change the rate of their period.  Other LPV stars that show steady or rapid changes are stars such as R Aql, R Hya, R Cen and R Tau showing period decreases and stars such as T Umi, BH Cru, LX Cyg and W Dra showing period increases.  A number of “LPV of the Month” articles have been written about some of these in the last two years.  You might find them interesting reading.

Templeton et al suggested in their paper on “Secular Changes” that R Aql and R Hya are possible examples of thermal pulsing stars.  Woods and Zarro mentioned in their 1981 paper that R Aql’s behavior could reasonable interpreted “as the result of radius variations after the peak luminosity in a thermal pulse”.

Thermal Pulse

The Wikipedia article at https://en.wikipedia.org/wiki/Asymptotic_giant_branch#Late_thermal_pulse defines a “thermal pulse” this way:

“As many as a quarter of all post-AGB stars undergo what is dubbed a "born-again" episode. The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen. If the helium is re-ignited a thermal pulse occurs and the star quickly returns to the AGB, becoming a helium-burning, hydrogen-deficient stellar object.  If the star still has a hydrogen-burning shell when this thermal pulse occurs, it is termed a "late thermal pulse". Otherwise it is called a "very late thermal pulse".

The outer atmosphere of the born-again star develops a stellar wind and the star once more follows an evolutionary track across the Hertzsprung–Russell diagram. However, this phase is very brief, lasting only about 200 years before the star again heads toward the white dwarf stage. Observationally, this late thermal pulse phase appears almost identical to a Wolf–Rayet star in the midst of its own planetary nebula.”

Some LPVs evolution models suggest that an LPVs period may decrease at the beginning of a thermal pulse (TP) from a helium flash and the period may increase later on in a thermal pulse (TP) so it's possible that R Aql is in a state not long after the thermal pulse. 

There is much discussion on thermal pulses in general and if they really cause this type of behavior that are beyond the scope of this article.

Hump/Standstill

R Aql does show an occasional “hump” or “stand still” on the ascending part of it’s Light Curve (LC).  It shows up quite well in the Visual data points for some cycles.   For more on “humps” or “stand stills” of LPV stars, please view the “LPV Humps” page of the “AAVSO Long Period Variable Section” web page.

Observations

One thing that is sorely missing in the data points for R Aql is “color” information for the star.  Because of it’s relative brightness, trying to get date using a CCD Rc or Ic photometric filter can be challenging particularly when the star is near it’s maxima.  As an example, the Ic magnitude for R Aql is around 4th magnitude when the star is at it’s minima!  I’m not sure what the magnitude would be at maximum because there is no value in the AID for Ic at maxima.

At the minimum, observations with a CCD camera should be taken with a V and B filter and transformed which would give some “color” information about the star along with the wonderful visual  data that is collected.

R Aql may be bright enough to try to get some spectrum of the star which might be helpful gathering further information as the period of the star continues to change.

This LPV is one of a very small group of LPVs that show large period changes that should be followed by AAVSO and other observer for another 150+ years. I suspect that it may provide many surprises for us as it goes through further changes.

References and further reading

J. Greaves & J. J. Howarth, “Further investigations of R Aquilae”, 2000, Journal of the British Astronomical Association, 110, 3, 2000 pages 131- 142

Templeton, M., Mattei, J.A., Willson L.A., “Secular Evolution in Mira Variable Pulsations”, 2005, The Astronomical Journal, 130, 776

Uttenthaler, Stefan et al, “The evolutionary state of Miras with changing pulsation periods”, 2011, Astronomy & Astrophysics

Wood P. R. & Zarro D. M., ApJ, 247, 247-256 (1981)

 


 

SEPTEMBER 2017

T Camelopardalis

T Cam is a bright Mira that has been well observed by the AAVSO since 1904.  Over 18,000 observations are in the AID from over 600 different observers as can be seen in the light curve shown below which displays all observations in the AID.  It is one of the Legacy LPV Program stars.  Since it’s a northern circumpolar star, there is generally very good coverage year round

The light curve based on the last 2,000 days is shown below.

 

The light curve based on the last 6000 days is shown below.

It's spectral class is S4 to S8.5 according to the VSX and is classified as a Mira.  The GCVS lists T Cam as having a visual magnitude range of 7.3 to 14.4 with a mean period of 369.3 days. 

Analysis of all AID data in the period 1904 to the present day indicates an overall mean period of 373.6 days.  There is also another signal present at 357 days which could be an annual alias of the main period.  Another one is present at 185 days.  This one could be an annual alias, however since coverage of the star over the year is reasonably complete, it is more likely this is a harmonic of the main period which reflects the non-sinusoidal nature of the light curve. 

A reasonable amount of CCD photometry has been reported to the AID since 2003.  B-V, V-R and R-I have been measured around 2.0, 2.0-2.5 and 2.0-2.7 respectively at both around minimum and maximum. 

T Cam is contained in the list of LPV Humps maintained by Frank Schorr as a possible “candidate” for a LPV hump star.  It seems to show what appears to be a double maximum on specific cycles.  This is evident to some degree in the phase diagram below which is based on the complete dataset.  For other cycles, the discontinuity appears to be more a hump on the ascending part of the lightcurve rather than a dual maximum

The plot below provide a closeup of a dual maximum feature for a specific cycle (early 2004).

The plot below provides a closeup of a hump feature for a specific cycle (mid 2005).  The difference between the dual maximum and hump features appears to be that there is a definite dip in brightness between the discontinuity and the main maximum for the double maximum case.

Marsakova and Andronov (2007) analysed a selected group of LPV's which show hump features in their light curves based on 80 years’ worth of data from the VSOLJ and AFOEV and T Cam was one of the stars analysed in greater detail.  They found that, as with many other LPV's with humps, the humps don’t occur in every cycle.  For T Cam, they found that it occurred on in 2 out of 3 cycles on average (68 cycles in 100 to be more precise). 

The average phase of the cycle where the dual maximum or hump appeared is 0.16.  This can be seen in the phase diagram above based on AAVSO observations.  T Cam has an asymmetrical light curve with the ratio of duration of ascending branch to the total period being about 0.476.

An analysis has been performed to ascertain any changes in mean period in T Cam over the past 113 years based on observations in the AAVSO AID.  The figure below suggests that there has been a "meandering" of the mean period between the range of 366 to 380 days.  This slow meandering behaviour (with period changes generally less than 3.55% of the mean period) is relatively common in the longer period Miras and semi-regular stars.  Similarly there is a small change in the amplitude as shown in the Figure below.

 

Interestingly T Cam was once believed to show remarkably large proper motions (over 0.1” per year) and was out of kilter with the proper motions measured for other S-type stars.  Stephenson and Hulbert (1984) redetermined T Cam’s position on historical plates and came up with a much slower rate of progression of around 0.009” per year maximum thus revealing it is quite normal!

T Cam is an interesting LPV to follow so all are encouraged to maintain the coverage that has been in place over the last 113 years.

References

"Variability of Long Period Variable Stars III: Changes in the Parameters of Humps in the Ascending Branch", V.I Marsakova & I.L Andronov, Astrophysics, Vol 50, No. 1, 2007

“Correction of Spuriously Large Proper-Motion Determinations for the S-Type Stars T Cam and S Uma”, C.B Stephenson & S.J Hulbert, Astron. J, 89, 1261 (1984)


 

AUGUST 2017

CT Lacertae

CT Lacertae (CT Lac) is a SRB type Long Period Variable (LPV) that has @1200 observations Visual and CCD  B, R, R and Ic bands in the AAVSO International Database (AID).

CT Lac (R.A. 22h 06m 39.93s, Dec. +48° 27' 06.9" (J2000)) was discovered by Morgenroth (1936) and was noted as an “irregular” variable at the time.  The star is well placed most of the year for viewing.

VSX shows CT LAC as a SRB type LPV but there are also references to CT Lac as a SRA type LPV

SRB LPV type

The General Catalogue of Variable Stars (GCVS) defines the SRB LPV class as:

“Semiregular late-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20 to 2300 days) or with alternating intervals of periodic and slow irregular changes, and even with light constancy intervals. Every star of this type may usually be assigned a certain mean period (cycle), which is the value given in the Catalogue. In a number of cases, the simultaneous presence of two or more periods of light variation is observed.”  CT Lac does seem to show two periods.

Spectrum

CT LAC is an SRB type LPV variable of Spectral type C6-,4, a late carbon star, with the “C” denoting a carbon enriched spectrum. 

Carbon Star

From Wikipedia “A carbon star is a late-type star similar to a red giant (or occasionally to a red dwarf) whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance.”  It's this dark red appearance that should make this star a pleasure to visually observe!

It appears that CT Lac has a significant amount of carbon dust absorption.

Period

VSX has CT Lac’s primary period as 558.2 days with a secondary period of 286.3 days (almost half of it’s primary period).  (I have also seen 562 days for CT Lac’s period but this value has appeared to have been refined by Templeton et al).  This is a long period for a LPV but not unusual for a Carbon star.

Magnitude

It's Magnitude Range is typically 8.6 – 13.4 in Visual and V band.  This makes it particularly well suited for Visual observers and work in B, V, R and Ic for CCD observers.                                                                                                                

Dimming or Fading

This is the full Light Curve (LC)  graph for all of the Visual and V band observations in the AID which clearly shows the “dimming” event CT Lac is currently going though.  There may have been another dimming event in the past but there is not enough data in the AID to make a judgement about this having happened.

Closer look at the dimming event

Just before the latest dimming event started, the magnitude of  CT Lac at Maxima was at 10.5 in Visual on March 2008 .  Later that year CT Lac minima was 13.4 in Visual.

At the lowest point in the dimming event CT Lac had a Maxima of 12.6 Visual on October 2013 and a minima just before that of 14.2 in Visual October 2013.

The current for Minima of 13.8 Visual during Jun 2016 and Maxima of 11.5 during Oct 2016.

The lowest point of the Maxima during the dimming period was @2.5 magnitude dimmer then at the beginning of the dimming event and the Minima was @.8 Visual Magnitude dimmer at the low point then at the beginning of the event.  The Maxima values showed the greatest dimming.

CT Lac’s Minima and Maxima magnitudes appear to be climbing back toward the values it showed for 2008 and earlier however they have not fully recovered as of the date this article was written.  In the V band the differences appear to be even greater.

The length of the time of this dimming event is @3340 days or @9 years and the star may not have completely recovered yet, Perhaps it’s LC will continue to “fall apart” as T Umi is currently doing.  (Please see my “July 2016 LPV of the Month” T Umi article for more detail.)

Cyclical dimming behavior is seen in some other Carbon and semi-regular stars RU Vir and V Hya.  It’s felt these dimming events may be caused by dust and soot the star is throwing off or events dragging more up from lower in the star or for some unknown reason.

In my article on October 2016 LPV of the Month article on S Cep, I mention that S Cep is also a “carbon” star and has had a number of dimming events over time and at time has showed hints of Dual-Maxima (DM).

Dimming may have be caused high mass loss that may become optical obscured (dim) the star for at least a period of time.  Further observations over time are need to see if this pattern repeats in the future.

There may have been earlier dimming events for this star but there isn’t enough data in the AID for me to see an earlier event.  Observations will be needed in the future to determine is this is a cyclical event

Dual or Double Maxima

This light curve shows the visual data for CT LAC for the last 11.5 years and clearly shows the Dual or Double Maxima (DM) behavior of this star.  Although this behavior is not as clear to me earlier in it’s Light Curve (LC), CT Lac’s DM was reported around 1980.

 

Visual observers

This star is well placed for many visual observer, many of whom have tracked this star and given us the wonderful LC curve above showing both the dimming of the star and it’s Dual-Maxima (DM) behavior.

 Matthew et al in his paper also made the following point about doing CCD observing of this star, “Instrumental data would provide additional useful information, especially if Johnson B and V data were taken and properly transformed so that both the overall V-band brightness and the (B−V) color could be accurately tracked with time.”  This would help determine the nature of the DM seen in the star along with the “dimming” event along with Ic band images if possible to help separate the two portions of the Dual-Maxima feature.

CT Lac may be one of the stars that helps us understand the evolution of LPV stars and should have Visual of CCD observation for many years to help unfold more of it’s secrets.

For further details about CT Lac and where I pulled my inspiration for this article, please read the follow paper by Matthew Templeton, et al

Further reading:

Templeton, M. R., Maurer, P., Kriebel, W., Lowder, W. M., Morelle, E., O’Connor, S., Arminski, A., Bichon, L., Bortle, J. E.,  “CT Lacertae: Another Long-period Carbon Star with Long Timescale Variations?”, 2014, The Journal of the American Association of Variable Star Observers (JAAVSO) volume 42, p. 260

AAVSO Forum discussion:  https://www.aavso.org/ct-lacertaes-recent-decline

 


JULY 2017

T Cassiopeiae

T Cas is a bright Mira that has been well observed by the AAVSO since 1870 with over 54,000 observations in the AID from over 1,500 different observers.   The light curve shown below displays all observations in the AID.  It is one of the Legacy LPV Program stars.

The light curve based on the last 2,000 days is shown below.

The light curve based on the last 1,000 days is shown below.

 

 

Its spectral class is M6e to M9e according to the VSX and is classified as a Mira.  The GCVS lists T Cas as having a visual magnitude range of 6.9 to 13.0 with a period of 440 days. Analysis of all AID data in the period 1870 to the present day indicates an overall mean period of 444.2 days.  The analysis also clearly shows smaller harmonics in addition to the 444 day fundamental period.  These occur at periods of 222 and 148 days which were also found in the analysis performed by Howarth (1997).

It displays an asymmetric light curve with an F value of 0.44 (where F is the fraction of the period that maxima occur after the previous minima). 

The light curves seem to show a pronounced double peak or maximum in a number of cycles.  This can be seen in the phase plots shown below.  However recently observed double maxima are more clearly shown when considering the CCD observations.  Howarth (1997) also reported the presence of humps on the ascending branch of the light curve.  Based on nearly 12,000 observations made by the BAA VSS in the period between 1920 until 1995, the hump was visible on many of the cycles and the double maximum was prominent in 1920-1921, 1939 and 1987-1988.  This is confirmed in AAVSO data as well with another less well observed double maxima possibly occurring in 1945 and others in 1961 and 1978.  Since 1995 there appear to have been further double maxima in 1995-1996, 2004-2005, 2007, 2008, 2011 and 2015.

A particularly pronounced double maximum, which was observed in 1939 is shown in the figure below.

Howarth (1997) analysed changes in the amplitude and phase of the fundamental periods and the harmonics and found that when the phase of the first harmonic retreats compared to the fundamental period, this shows up in the light curve as a double maximum feature.  When the phase of the first harmonic advances compared to the fundamental period, this shows up in the light curve as the hump moving along the ascending branch away from a double maximum feature. 

Howarth and Greaves (2001) surmised that the hump on the ascending branch of the light curve migrates with a cyclical period of approximately 3,000 days and that this variation is related to changes in phase of the first harmonic.  The amplitude of the first harmonic and the phase difference between the primary period and first harmonic varies in a time frame of approximately 3,000 days. During this migration, at certain points, the hump manifests itself in the appearance of the double maximum.

Mathew Templeton’s poster paper on Miras with double maxima also contains an interesting discussion on possible causes.

Stan Walker from RASNZ VSS provided an interesting commentary that the vast majority of currently identified LPV’s which display the double maximum feature are located in the southern sky.  Furthermore these are all located in a small region of the sky (1.7% of the total sky).  T Cas currently appears to be the only other candidate located in the northern hemisphere.  It would be a worthwhile project to look for other DMM stars outside of the currently identified group.

T Cas was one of the Miras studied by Lockwood and Wing (1971) in the near infrared.  The IR light curve displayed the same general features as can be seen in the visual light curve including the humps on the ascending branch.  They also found that the various features of the light curve seem to lag the visual features at the longer IR wavelengths.  In the case of T Cas, this amounted to about a 10% of the period lag.

Only limited CCD photometry has been reported to the AID.  B-V has been measured in the range of 1.5-2.0 throughout all phases of the light curve.

T Cas is also believed to be one of the nearest known AGB stars

An analysis has been performed to ascertain any period changes in T Cas over the past 140 years based on observations in the AAVSO AID.  The figure below suggests that there has been a "meandering" of the mean period between the range of 439 to 449 days. This slow meandering behaviour (with period changes general less than 5% of the mean period) is relatively common in the longer period Miras and semi-regular stars.

 

 

T Cas is an interesting LPV to follow so all are encouraged to maintain the coverage that has been in place over the last 140 years.

References

"T Cassiopeiae – a predictable variable?", J. J Howarth JBAA, 107, 5, 1997

“Light curves of Mira Variables at 1.04 Microns”, G.W Lockwood & R.F Wing, The Astrophysical Journal, 169:63-86, 1971

“Amplitude and phase changes in the light curves of long period variables”, J.J Howarth & J. Greaves, MNRAS, 325, 1383-1388 (2001)

Templeton poster - https://www.aavso.org/files/publications/staff_pubs/templeton_mira_humps_bumps.pdf


JUNE 2017

BH Crucis (BH Cru)

Summary

VSX shows that BH Cru is a Long Period Variable (LPV) star of type Mira and can be found at 12 16 16.79 RA   -56 17 09.6 DEC (328.98762 +48.34792) which makes it a star only for southern observers.

The magnitude range for this star in V is 6.55 – 10.1 which makes it a good target for both visual and CCD observers.

Since it was discovered in 1969 by R. G. Welsh, BH Cru's period has increased from about 421 days in 1970 to about 525 days around 2000.  Since 2000 BH Cru's period has slowly but steadily decreased to about 505 days currently and may now be stable.

During this same time period BH Cru's Spectrum has changed from an SC star to a CS or C (carbon) star.

BH Cru had Dual-Maximas  when it was discovered but since then this attribute has disappeared with the period increase and currently it just shows a “hump” or “stall” on it's ascending Light Curve (LC)

Observations

 

The first observation for BH Cru in the AAVSO International Database (AID) is from 1969 just after it was discovered.  Currently the AID database has 9360 observations in visual and multi-bands.  Many of the observations have been made by visual observers.

Dual-Maxima

As seen in the light curve (LC) above, BH Cru had a strong Dual-Maxima (DM) for a number of cycles after it was discovered up in 1969 up to about cycle 13.  As the period of BH Cru increased the DM behavior disappeared and today there are only single but noticeable “humps” or “stalls” on the ascending part of the LC.  This same behavior has been seen in star R Cen.  It appears that the “hump” for BH Cru may be derived from the first maxima when the BH Cru showed the Dual-Maxima behavior. 

Period Change

The above is a period graph for BH Cru from Thomas Karlsson's monumental work of the O/C, Period and Mean Lightcurves of around 490 Mira variable stars.  The main webpage of his study can be found at http://var.astronet.se/mirainfooc.php.  

This graph shows BH Cru had a period of 421 days when it was discovered in 1969 and steadily increased in period to about 525 days around 1999, an increase of 104 days in only 12 cycles as pointed out by W.S. G. Walker in a paper he wrote on changes in BH Cru.  Since about 2001, BH Cru's period has decreased at a slower rate until it's about 505 days in length today.

This behavior is almost identical to period changes for star LX Cyg which is described in an earlier “LPV of the Month” article and might be worth reading again because of the simularity of these two stars;

Some LPVs evolution models suggest that an LPVs period may decrease at the beginning of a thermal pulse (TP) from a helium flash and the period may increase later on in a thermal pulse (TP) so it's possible that BH Cru is near the end of a thermal pulse (TP).  However papers by Zijlstra et al and Walker argue that BH Cru has not had a recent TP.

Spectrum Change

When BH Cru was first discovered it's spectral type was measured to be between spectral types SC4.5/8-e and SC7/8-e (showing ZrO bands) which is rare amoung LPVs..  By 2004, BH Cru's spectrum evolved into a CS (Showing weak C2) spectrum and perhaps a C (carbon) spectrum today.  This change occurred during the dramatic period  increase for this BH Cru.  A paper by Zijlstra et al discusses this change in great detail and is certainly worth reading.

Observing

BH Cru may have many more surprises for us as time moves on.  It's a good star to add to your observing list if you are a southern observer or have remote access to a telescope that is in the southern skies.  For CCD observers it has been suggested that the observations be made using the B, V, Rc, Ic filter set and U if available.  B and V observations would be the most useful for ongoing projects and be transformed if possible but observations in the other bands/filters are important too.  This star still has a number of unanswered questions about it's behavior!

References and further reading

Templeton, M., Mattei, J.A., Willson L.A., “Secular Evolution in Mira Variable Pulsations”, 2005, The Astronomical Journal, 130, 776

Uttenthaler, Stefan et al, “The evolutionary state of Miras with changing pulsation periods”, 2011, Astronomy & Astrophysics

W.S. G. Walker, “BH Crucis: Period, Magnitude, and Color Changes”, 2009, JAAVSO, Volume 37, Pages 87-95

Zijlstra, Albert A. et al, “Period and chemical evolution of SC stars”, 2004, Monthly Notices of the Royal Astronomical Society


 

MAY 2017

R Hydrae

R Hydrae is an M (Mira) type of LPV of Spectral type M6e-M9eS(Tc).  It's Magnitude Range  3.5 to 10.9 in both V and Visual.  Currently there are 24,890 observations in the AAVSO International Database starting in April 5, 1842 although there are reports of observations going back to 1662.  R HYA is located at 13h 29m 42.78s and -23d 16m 52.8s which means it can be observers from much of the Northern and Southern hemisphere.  It is also bright enough that it should be a good object for Visual observers.

R HYA (R Hydrae) was the third Mira-type variable star to be discovered in the early 1700s.

An excellent article on the background of R HYA can be found on the AAVSO website at https://www.aavso.org/vsots_rhya.  This article is to provide an update to the above article.

Period Decline

R Hya is another Mira star that is showing a declining period. It's gone from about 495 days around the year 1700, 480 days by 1800 and 450 days in 1850.   By 1920 the period was 420 days and in 1950 it slowed down and  started to vary between 380 to 395 days.

 

The above is a period graph for R Hya from Thomas Karlsson's monumental work of the O/C, Period and Mean Light Curves of around 490 Mira variable stars.  The main webpage of his study can be found at http://var.astronet.se/mirainfooc.php and graphically shows R HYAs period decline.  It has been mentioned that this period decline “leveled” off around 1950.

 

However from this Period graph it looks like the period for R HYA is declining again since the mid-90s to about 370 days and dropping.  It is important to watch the period of this star in the future to see if this star's period continues to drop, particularly in Visual and V.  This behavior may be the result of a “shell flash” and more data in the future may help astronomers understand what the long term light curve may look like after a “shell flash” occurs in an LPV.

 

Observations

The above graph taken with AAVSO VSTAR tool shows all of the Visual and V observations reported to the AAVSO International Database going back to 1842.

The magnitude of R HYA's Maxima and Minima can vary like many other Mira stars.  In this closer look at R HYA's light curve it's been mentioned that since 2008 the over all brightness for each cycle seems to have leveled off and now appears to be getting smaller.  It appears that this behavior for R HYA may happened a few times in the past.  It has been suggested that this behavior may be loosely related to the star's period decrease but needs to be watched in Visual and V filter to see if the behavior continues.

Shell Flash

John Isles in a 1996 paper does an excellent job explaining the potential “shell flash” that R HYA may have gone through to product it's decreasing period.  I quote his writings from the earlier AAVSO article mentioned above.

“The century of steady decrease in R Hydrae's period is consistent with theoretical calculations of what should happen to a pulsating red giant after the sudden ignition of helium near its core. Mira stars are old and highly evolved, and their supplies of nuclear fuel are close to exhaustion. At the center of the star is a small but very dense core composed almost entirely of carbon and oxygen, the end products of helium fusion. In a shell just outside this core, hydrogen is being converted to helium, so a thin layer of helium builds up on the core's surface. Every 1,000 to 10,000 years this helium shell should reach a critical mass and ignite, creating more carbon and oxygen.

As soon as helium burning begins, the shell rapidly expands and the hydrogen burning above it turns off. For about a century the star derives its energy from helium fusion. When most of the helium is consumed the "shell flash" ends, the shell shrinks, and hydrogen burning resumes. A star may undergo several helium-shell flashes near the end of its time as a red giant. Eventually the star's outer layers will be expanded into space, and the carbon-oxygen core will shine as a white dwarf.”

Zijlstra et al. (2002) suggested that R HYA's behavior could be part of “Post thermal pulse evolution” or “Envelope relaxation” details of which can be found in their paper.

Bow-shock

The NASA Spitzer Space Telescope found that there is a “bow-shock” caused by the stellar wind from R HYA hitting and pushing against gas and duct between stars.  This maybe the first “bow-shock” seen associated wit a Mira. Please see https://photojournal.jpl.nasa.gov/catalog/PIA09070http://kencroswell.com/RHydraeBowShock.html and https://arxiv.org/pdf/astro-ph/0607303.pdf for further information.

Isles, John E. "R Hydrae's Helium-Shell Flash." Sky & Telescope, May 1996, 68-70.

Ueta, T et al “Detection of a Far-Infrared Bow-Shock Nebula around R Hy: the First MIRIAD resylts”

Zijlstra, Albert A., T.R. Bedding, and J.A. Mattei. "The Evolution of the Mira Variable R Hydrae." Monthly Notices of the Royal Astronomical Society, to be published in 2002


 

APRIL 2017

V Hydrae

  1. Introduction

April’s LPV is V Hydrae (V Hya), a carbon star Mira that is exhibiting evolutionary changes that are rapid enough to be detected and followed in human lifetimes. V Hydrae’s light curve reveals a periodicity that is likely due to it being a Mira pulsating star with a much longer superimposed period that is likely due to outflows from the stellar envelope that interact with a binary companion star. These phenomena contribute to much deeper minima at intervals of about 17 years than would be expected from a Mira variable.

Stellar evolution is excruciatingly slow when compared to most other time scales, particularly those of human lifetimes. This makes it quite frustrating to those who study the subject and have to endure the reality that observational evidence of stellar birth through death is difficult to detect. However, there are rare occasions when a star encounters a stage of evolution that proceeds much faster than usual, and this can be studied in decades or years. Finding such stars is a boon to astronomers and astophysicists who, through observation, can learn what is going on and try to tie this knowledge to a star’s overall evolution. Variable star observers have opportunities to greatly contribute here by adding their brightness measurements to the light curves that are much used by the professionals studying these stages.

V Hya is such a star. Its light curve behavior has led to recent studies that have resulted in some interesting conclusions about what is occurring with this object and its evolutionary state, and what this implies for other highly evolved stars of similar mass. This is ultimately due to the observations by AAVSO members and other variable star watchers. V Hya is well worth adding to one’s observing program and continuing observation.

V Hya offers a rare opportunity to observe the behavior of a star near the end of its evolution on the asymptotic giant branch (AGB). Because of the relative rapidity of this behavior it is difficult to find such stars. So, we must take advantage of what V Hya is telling us while we can.

  1. About V Hya

V Hya (RA 10h 51m 37s, Dec -21 15’ 0” (J2000)) was suspected of variability in the early 1870s; this was confirmed shortly thereafter. The magnitude range was estimated to be 7 to 8.5 with a period of about 535 days. In 1905 the magnitude range remained the same but the magnitude was decreasing, i.e., becoming fainter. In 1908 the star reached a deep minimum magnitude of 12, then began to recover and gradually brightened. The period remained the same during this minimum as when brighter. Other deep minima during these early years of observation occurred in 1924-26, 1942-43, and 1959-63. Data is scarcer but there is an indication of a deep minimum in 1891. These occurred at intervals of about 17 years. (Mayall 1965)

Today V Hya is classified in the General Catalog of Variable Stars (GCVS) as a semiregular pulsating variable of type SRa. It exhibits a pulsation period of 530 30 days. This is rather long for most stars of this type but because of the regularity of this period and V Hydrae’s status on the AGB this object is more properly classified as a Mira. The visible brightness amplitude due to pulsation is 1.5m. There is a much longer superimposed periodicity of 6160  400 days (about 17 years), more of which later (Knapp et. al. 1999).

V Hya is also a carbon star of spectral type C7,5e(N6e). This indicates a luminous, cool red giant in the Morgan-Keenan carbon star spectral classification system. The “e” indicates emission lines are sometimes seen against the predominant absorption line spectrum.

The star’s effective temperature (Teff) is 2650K. Its bolometric luminosity is approximately 1.4 x 104 solar. Its distance is estimated at roughly 500 pc.

V Hya has a circumstellar envelope that may be somewhat flattened, and it exhibits a strong, fast stellar wind (and thus a large mass loss rate). Because of this V Hya may be in an early stage of evolution off the AGB. The star then would be in a pre-planetary nebula formation stage (Knapp et. al. 1999). This makes it a star of great interest for continued observation and study.

V Hya has two companions in a multiple star system. One is a 11.0m K0 type giant at a distance of 46” and the other is an unseen star with a period of, interestingly, 8.5 years (half of 17!) (V Hydrae Wikipedia).

  1. The Light Curve  

Figure 1 shows V Hydrae’s light curve as derived from the earliest observations in the AAVSO database to February 2017. The curve includes observations dating to about 1905 and continuing to the present. The star has been fairly well observed since about 1920. The curve is generated using AAVSO’s enhanced light curve generator (enhanced LCG), an online tool for observers at AAVSO’s website.

There is notable scatter in the data due at least to the Purkinje effect (the changes in the eye’s response to color under low light levels; a very red carbon star will look brighter the longer it is observed and this makes accurate visual estimates with other color comparison stars difficult).

Even a casual examination of the curve reveals why this star is so interesting. Most years V Hya shows rather typical Mira star behavior although its period of 530 days is longer than usual for most Miras (periods of around 300 days). Also, the magnitude range of its variation is largely the same. What are noteworthy are the deep minima that occur at intervals of 17 years. These minima do not reach the same magnitude each time but are still quite a bit fainter than those at less than 17 year intervals.

The star’s fading and recovery of brightness is abrupt and fast, in step with the pulsation period of 530 days. Indeed, this period does not seem to be affected by the deep minima occurrences.

Fig. 1. The complete light curve of V Hya from all AAVSO data. For convenience the timeline is in calendar date, not the usual Julian Date.

The deep minimum of 1994 was well followed by the observers and V Hya faded to below the 13th magnitude. The deep minimum of 2011 was not as well followed but the indication is of a more “normal” deep minimum.

Figure 2 is the AAVSO light curve from 1960 to the present. It shows more detail than figure 1 of the pulsation period and the last four deep minima.

 

Fig. 2. The light curve of V Hya from 1960 to February 2017. The magnitude variations can be seen in some detail. Again, the time axis is presented in calendar date.

The more recent light curve is presented in Figure 3. The data covers the present time and goes back about 750 days. This curve, between deep minima, illustrates the behavior of a “normal” long period Mira. The green data points are of photoelectric or CCD photometry observations (as also in the previous curves). Despite some data point scatter (and easily excluded from analysis) visual observations do a rather good job of following the variations.

 

Fig. 3. The light curve for approximately the past 750 days to present showing more typical long period Mira behavior.

  1. Why V Hydrae’s Behavior

The noteworthy behavior of V Hya is the deep minima that are seen in the light curve about every 6200 days (17 years). Similar behavior – that of a long superimposed variation on a Mira variable – is seen in a few other Miras also (e.g., CT Lac and RU Vir). However, these other stars have not been observed as long as V Hya. Thus, it is not known how periodic these variations may be.

Current research strongly suggests that at least a partial cause for V Hydrae’s and other similar Miras long period variations of dimming is due to a considerable outflow of material from the star as each begins the early stages of a “superwind” that marks the transition from the AGB to the post-AGB phase of evolution (Knapp et. al. 1999, Templeton et. al. 2014 and Willson 2000). Studies of V Hya at other wavelengths, primarily at millimeter wavelengths have revealed the large outflow of cool gas from the star’s outer layers. Thus, the changing circumstellar environment due to rapid evolution is also a factor in the variable’s behavior.

Because of the 17 year periodicity, though, researchers suspected something else in addition to gas outflow from the star is causing it.

Researchers began observing V Hya in the 1980s to learn more about the star’s circumstellar environment. Their observations have been principally in the millimeter wavelengths that allow them to study molecules in the outflow such as CO. One of these research groups, led by R. Sahai of the NASA Jet Propulsion Laboratory, discovered “blobs” of gaseous material that have been apparently ejected from V Hya. These blobs (or “cannonballs” as they have been described) move at high velocities and the outermost ones are were ejected approximately 400 years ago. The “cannonballs” look to have been “fired” at intervals of 17 years. The next step was to discover the mechanism by which the gas blobs were “fired” by V Hya.

Sahai et. al. recently published the results of their work to find the mechanism. They propose V Hya has an unseen companion star with an eccentric orbit and a period of 8.5 years. The companion has an accretion disk around it made up of V Hya’s outflow. At the companion’s closest approach to V Hya it rapidly accretes the primary’s outflow. To maintain its stability the disk ejects matter (the plasma “blobs”) along the disk’s axis. If the blob is ejected to the “front” of V Hya (in the general direction of our line of sight) it is dimmed. However, the disk is believed to wobble somewhat with an 8.5 years period and that leads to the ejection of matter “behind” so that the primary’s light is not appreciably faded.

This theory fits well with observations of many planetary nebulae and their frequently seen complex structure. Current thought is these structures are caused by companion stars interacting with the mass outflows of the aged primaries. Accretion disks gather up this mass and eject it into space in various patterns apart from that of the primary alone. V Hya indeed looks to be at the beginning stage of forming a planetary nebula and becoming a white dwarf and at the end stage of a pulsating red carbon giant on the AGB.

Figure 4 (JPL News 2016) illustrates the present theory of V Hydrae’s variability. The caption provides additional details of the accretion disk’s role in the 17 years cycle of deep minima.

 

 

Macintosh HD:Users:msoukup:Downloads:V Hya Plasma Ejections caption.PNG

Fig. 4. Graphic illustrating V Hya and its companion ejecting plasma  “blobs” into space at 17 year intervals via an accretion disk around the companion. Courtesy NASA and the Jet Propulsion Laboratory, 2016.

This work has made use of the AAVSO International Database and the AAVSO International Variable Star Index, operated at the AAVSO, Cambridge, MA, USA. It has also made use of the SIMBAD database, operated at CDS, Strasbourg, France.

References

AAVSO, 2017, observations from the AAVSO International Database and Enhanced Light Curve Generator (http://www.aavso.org)

Mayall, M., Variable Star Notes, 1965, R.A.S.C. Jour. 59, no. 5, 245

NASA/Jet Propulsion Laboratory, News, October 6, 2016 (http://www.jpl.nasa.gov/news/news.php?feature=6639)

Sahai, R. et. al. 2016, ApJ 827, 92S

Templeton, M.R., et. al. 2014, JAAVSO 42, 260

Wikipedia, V Hydrae (http://en.wikipedia.org/wiki/V_Hydrae)

Willson, L.A., 2000, Ann. Rev. Astron. Astrophys., 38, 573


 

MARCH 2017

R Lep

R Lep is a bright Mira that was discovered by the British astronomer J.R Hind back in 1845.  It became known as Hind’s Crimson Star due to the striking red colour of the star.  Hind described it as “resembling a blood drop on the background of the sky”.  There are observations in the AAVSO AID dating back to 1853.  Over 22,000 observations are in the AID as can be seen in the light curve shown below which displays all observations in the AID.  It is one of the Legacy LPV Program stars.  It was well observed starting from 1853 until about 1879.  There are breaks in the data where no observations were made in the period 1879 to 1893 and then between 1895 to 1905.  It has been well observed since the establishment of the AAVSO in 1911.

The light curve based on the last 2,000 days is shown below.

The light curve based on the last 700 days is shown below.

The last light curve indicates that there is a reasonable degree of scatter associated with visual observations of R Lep.  This is due mainly to the very strong reddish colour of the star which is easily apparent throughout it’s full light cycle.  Only a few B measurements have been made of R Lep as reported in the AID.  However B-V is typically between 4 and 5 magnitudes which indicates a very cool red star.

There is a lot of good guidance and discussion on how to successfully observe very red stars on the AAVSO web site and the reader is directed to https://www.aavso.org/determining-carbon-star-magnitude-direct-v-averted and https://www.aavso.org/estimating-red-variables.

It's spectral class is C7 according to the VSX and is classified as a Mira.  The GCVS lists R Lep as having a visual magnitude range of 5.5 to 11.7 with a period of 427.07 days.  However the GCVS does note that there have been changes in the star’s period.  Up until 1891, the GCVS states that the period was 436.44 days, between 1891 and 1921 it was shorter at 418.04 days, between 1921 and 1957, it increased to 441.38 days and since 1957 it’s period has been at the quoted duration of 427.07 days.   Analysis of all AID data using VStar in the period 1853 to the present day indicates an overall mean period of 436.7 days. 

A WWZ analysis has been performed to calculate any changes to the mean period.  The analysis has been restricted to observations between 1911 and the resent day and is shown in the Figure below.

The figure above suggests that there has been a "meandering" of the mean period between the range of 427 to 446 days. This slow meandering behaviour (with period changes general less than 5% of the mean period) is relatively common in the longer period Miras and semi-regular stars.

However probably the more interesting phenomenon that R Lep displays is the long term variation in the mean brightness.  The GCVS notes that the magnitude at maximum varies between a range of 5.5 to 9.5 and indicates that this occurs with a possible period greater than 40 years.  This variation in the mean brightness of R Lep is readily apparent in the first figure above. 

The figure below plots the mean magnitude of R Lep based on a bin size equal to it’s mean period (shown by the blue line).  It appears to vary by up to 3 magnitudes.  The long term modulation in the mean magnitude does appear to have a period of around 40 years, although clearly even a century long data set is insufficient to be more definitive.

This sort of behaviour has been displayed in other long period variable stars such as RU Vir, V Hya, R For and W Aql.  As noted previously by Matthew Templeton (https://www.aavso.org/vsots_ruvir) for carbon stars, the normal Mira-like variations seem to operate independently of the long term variations in mean magnitude.  Matt noted that this is a strong hint that the modulation is occurring outside of the star, rather than in the star itself where the pulsational variations occur. The best explanation seems to be a modulation in the dust opacity around the star. If a shroud of dust surrounding R Lep gets thicker or thinner we would see the star get brighter or fainter without seeing much of a change in R Lep’s underlying behavior. Matt also noted that T. Lloyd-Evans (1997) describes the occasional fadings of several well-known Miras, including R Lep.   His paper contains a plot showing the visual lightcurves of R Lep with the flux density of C2 (diatomic carbon), showing the visual fadings occurring at the same time that the infrared emission from C2 is increasing.

 

Raveendran (2002) carried out polarimetric observations of R Lep in the period 1991-2002.  This period coincided with a minimum in the mean magnitude of the star as shown in the figure above (around JD 2450000).  An analysis of the data shows that the percentage linear polarization increased as the fading progressed, attained a maximum of slightly over 3 per cent in the V band close to the epoch of minimum, and remained more or less at the same level during and well after the recovery to normal brightness. The polarization, apparently, originated from the circumstellar envelope above the region where the dust that caused the fading in the star condensed. The physical mechanism that causes the rather large polarization during fadings is perhaps selective extinction by aligned foreground grains produced by the passage of shocks through the circumstellar envelope.

R Lep is an interesting LPV to follow so all are encouraged to maintain the coverage that has been in place over the last 160 years.

References

Lloyd-Evans, T., 1997, " Spectroscopic changes and the variable mean light of carbon stars", MNRAS 286, 839

Raveendran, A.V., 2002 “Fading of light maximum and linear polarization variation in the carbon Mira R Leporis”, MNRAS, Volume 336 Issue 3 pp 992-998

 


 

FEBRUARY 2017

U UMi

U UMi is a bright Mira that has been well observed by the AAVSO since 1907.  Over 30,000 observations are in the AID from over 850 different observers as can be seen in the light curve shown below which displays all observations in the AID.  It is one of the Legacy LPV Program stars.

The light curve based on the last 2,000 days is shown below.

The light curve based on the last 700 days is shown below.

 

It's spectral class is M6e to M8e according to the VSX and is classified as a Mira.  The GCVS lists U UMi as having a visual magnitude range of 7.1 to 13.0 with a period of 330.9 days.  However the GCVS does note that there has been changes in the star’s period which appear to be oscillations between a maximum and minimum period.  Up until 1913, the GCVS states that the period was 323.1 days, between 1913 and 1940 it was longer at 331.3 days, between 1940 and 1953, it decreased to 320.6 days, between 1953 and 1953 it increased to 331 days, between 1958 and 1965 it decreased to 322.6 days and since 1965 it’s period has been at the quoted duration of 330.9 days.   Analysis of all AID data in the period 1907 to the present day indicates an overall mean period of 325.7 days. 

Only limited CCD photometry has been reported to the AID.  B-V has been measured around 1.5 at both around minimum and maximum. 

U UMi is contained in the list of LPV Humps maintained by Frank Schorr as a possible “candidate” for a LPV hump star.  It possibly shows a hump in the ascending branch of the light curve.  It’s difficult to see this in the phase diagram below based on the visual observations due to the relatively large scatter.

Marsakova and Andronov (2007) analysed a selected group of LPV's which show hump features in their light curves based on 80 year's worth of data from the VSOLJ and AFOEV and U UMi was one of the stars analysed in greater detail.  They found that, as with many other LPV's with humps, the humps don’t occur in every cycle.  For U UMi, they found that it occurred on average about every second cycle (51 cycles in 100 to be more precise). 

They also found that the average duration of the hump was 19 days which represents about 5.8% of the total period.  The average phase of the cycle where the hump appeared was 0.24.  The phase diagram based on AAVSO observations seems to indicate the phase is about 0.30 where the hump is observed.  U UMi has an asymmetrical light curve with the ratio of duration of ascending branch to the total period being about 0.54.

Since the hump duration only averages about 19 days in duration, it is very difficult to see these in the AID data if one considers the light curves above for the last 2,000 and 700 days.  So the jury is still out as to whether this star does indeed display definite humps.  Further observations at a higher cadence than normally conducted for LPV observations on the ascending branch of the light curve would be very useful to confirm the presence of humps.

An analysis has been performed to ascertain any period changes in U UMi over the past 110 years based on observations in the AAVSO AID.  The figure below suggests that there has been a "meandering" of the period between the range of 320 to 331 days.  Some of these changes replicate very closely the period changes prescribed in the GCVS mentioned above.  This slow meandering behaviour (with period changes general less than 5% of the mean period) is relatively common in the longer period Miras and semi-regular stars.

Percy et al (1999) confirmed that for almost all of the Miras in their large sample (which included U UMi), period changes in the stars are dominated by random cycle-to-cycle fluctuations in period, at least over intervals of about 20 cycles. This makes it very difficult to detect evolutionary period changes using simple O-C diagrams and there is one or more random physical processes going on within the star.

U UMi is an interesting LPV to follow so all are encouraged to maintain the coverage that has been in place over the last 110 years.

References

"Variability of Long Period Variable Stars III: Changes in the Parameters of Humps in the Ascending Branch", V.I Marsakova & I.L Andronov, Astrophysics, Vol 50, No. 1, 2007

“Long Term Changes in Mira Stars. I. Period Fluctuations in Mira Stars”, J. R. Percy & T. Colivas, PASP, 685, 111: 94-97 (1999)

 


 

JANUARY 2017

RS CYGNI

RS Cyg is a bright semi regular star that has been well observed by the AAVSO since 1890.  Over 42,000 observations are in the AID from over 1,300 different observers as can be seen in the light curve shown below which displays all observations in the AID.  It is one of the Legacy LPV Program stars.

The light curve based on the last 2,000 days is shown below.

The light curve based on the last 700 days is shown below.

 

 

 

It's spectral class is C8.2e according to the VSX and is classified as an SRa.  The GCVS lists RS Cyg as having a visual magnitude range of 6.5 to 9.5 with a period of 417.4 days.  Analysis of all AID data in the period 1890 to the present day indicates a mean period of 419.0 days.  This places RS Cyg in a large group of Miras and semi regulars with a long period (>350 days) which do generally show some interesting light curve characteristics.

SRa stars are pulsating variables in which temperature, ionization, and opacity oscillate to produce variable periods, amplitudes, and/or multiple periods.  These processes happen in the stars’ convective layers, and it is possible that convection plays an important role in their behaviour. In several SRa stars chaotic behaviour has been detected.  This includes RS Cyg.  Buchler et al (2004) and Binder et al (2008) specifically perform analysis of RS Cyg.  Firstly they were able to eliminate multiperiodicity for these pulsations for RS Cyg (i.e., they are not caused by the excitation of a small number of pulsation modes with constant amplitudes). They believe it is low dimensional chaos which explains the irregularity in the light curve.  They performed nonlinear analysis of the time series for RS Cyg and were able to postpredict the behaviour of RS Cyg to within 10% of the maximum variation in brightness in the light curve over a time horizon of approximately 16 months and further predict the future light curve for the next 16 months.

Only limited CCD photometry has been reported to the AID, however a couple of V and B measures indicate that it is a very red star (B-V = 4.0 at minimum).  This may explain the generally wide scatter in the visual estimates in combination with the bright range of the star.

RS Cyg is contained in the list of LPV Humps maintained by Frank Schorr.  It shows a hump (or possibly a dual maxima feature) in the ascending branch of the light curve.  It’s difficult to see this in the phase diagram below based on the visual observations due to the relatively large scatter, however its more clear if you focus on the V measures.

 

Marsakova and Andronov (2007) analysed a selected group of LPV's which show hump features in their light curves based on 80 year's worth of data from the VSOLJ and AFOEV and RS Cyg was one of the stars analysed in greater detail.  They found that, as with many other LPV's with humps, the humps don’t occur in every cycle.  For RS Cyg, they found that it occurred on average about every second cycle (56 cycles in 100 to be more precise).  They do note that the C stars they analysed (which includes RS Cyg) display noisiness in the light curve and other irregularities which does make it difficult to distinguish subtle hump or double maximum features that may be present. 

They also found that the average duration of the hump was 51 days which represents about 12.2% of the total period.  The average phase of the cycle where the hump appeared was 0.20.  The phase diagram based on AAVSO observations seems to indicate the phase is about 0.30 where the hump is observed.  RS Cyg has an asymmetrical light curve with the ratio of duration of ascending branch to the total period being about 0.60.

Marsakova and Andronov found a reasonably strong correlation between the brightness at which the hump generally occurs in a given cycle and the brightness of the preceding minimum and the following maximum.  For RS Cyg, these correlations were quite strong.  However their paper did not propose any physical significance of these correlations in terms of what may be causing the hump behaviour. 

An analysis has been performed to ascertain any period changes in RS Cyg over the past 125 years based on observations in the AAVSO AID.  The figure below suggests that there has been a "meandering" of the period between the range of 407 to 427 days.  This slow meandering behaviour (with period changes general less than 5% of the mean period) is relatively common in the longer period Miras and semi-regular stars.

 

RS Cyg is an interesting LPV to follow so all are encouraged to maintain the coverage that has been in place over the last 125 years.

References

"Variability of Long Period Variable Stars III: Changes in the Parameters of Humps in the Ascending Branch", V.I Marsakova & I.L Andronov, Astrophysics, Vol 50, No. 1, 2007

“Dynamics and forecasting of two chaotic stars”, P-M. Binder, et al, Astrophys. J., 685, L145-L148 (2008)

“Evidence for low-dimensional chaos in semiregular variable stars”, J.R Buchler, et al, Astrophys. J., 613, 532-547 (2004)

 

 

 

 

 
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