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LPV of the Month September, 2018

S Aurigae

A Carbon Star

Summary

This month's Long Period Variable (LPV) star, S Aur, should be a treat for Visual observers. S Aur is a fairly bright but very dark red Carbon star.  It’s B-V index is between ~3.5 to ~5, making it a very red object. (The B-V ratio seems to change over time, which might be investigated in the future with a focus on the B, V, and Ic bands.)

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.” 

Simply put, LPVs that have more Oxygen in their atmosphere than Carbon are M stars, LPVs with an equal amount of Oxygen and Carbon in their atmosphere are “S” stars (rare), and LPVs with more Carbon then Oxygen are "C" or carbon stars.  This seems to be the evolutionary path many LPVs take with few spending much time as an “S” star.  Further reading is required to understand how these changes occur.

In Dusek's paper (2003), he states: “Theoreticians claim that every star with the initial mass from 1.5 M⊙ to 4 M⊙ should go through the stage of a carbon star lasting about 3.105 years before it finally transforms into a white dwarf (Groenewegen et al., 1995).”  

It’s possible that the Carbon stage may in fact part of the evolutionary process for some LPVs of a certain size and perhaps the final stage before becoming a White Dwarf star.

S Aur is an SR (Semiregular variable star) type variable of spectral type C4-5,4-5(N3), with the “C” denoting a carbon enriched spectrum.  Its magnitude range is 8.2 to 13.3 in the Visual and V bands.  This makes it suited for Visual and CCD observers, depending where it is on its light curve (LC).  VSX reports that S Aur's period is 590 days, which is a long period for an LPV. 

S Aur can be found at RA (2000) 05 27 07.43  Dec (2000) +34 08 59.3, which makes it positioned well for Northern observers, as S Aur just coming into “season”.  It is a dark red star that is a delightful gem for the Visual observer.

S Aur's minimum and maximum magnitude are themselves variable over a period of decades, which is what makes this star and a number of other Carbon stars interesting! 

Observations

There are currently ~13,000 observations for S Aur in AAVSO International Database (AID) since April 26, 1904 (JD 2416596.6), making it a well observed star, particularly after the AAVSO was formed in 1911.

S Aur is currently part of the “LPV Program” stars and has had good observer coverage that should be continued, particularly with more color information.

Light curve

This is the complete Light Curve (LC) for S Aur in the AAVSO International Database (AID). It shows three major events for S Aur's “dimming” (the minima and maxima for the star getting dimmer over an extended number of periods) and the LC leading up until today that seems to be currently showing a “brightening trend” occurring. 

Details of the dimming events

This Light Curve (LM) uses a mean values of 500 days for its Visual observations.

The first event centers on ~2429882.  The dimming event seems to last 4 cycles or around 1350 days, with both the drop in the dimming and the rise back brighter being very steep.

The second event centers on ~2441410, or 11,528 days (31.5 years) later.  The dimming event may be some cycles longer then the first one and has a different looking profile with the drop into the dimming not very steep and a steep rise to brightening.

The third event may be centered on ~2453471, or ~12,000 days (33 years) later, with a drop into the dimming not very steep and a steep rise to brightening as seen in the second event.

The question that comes to mind is what causes this “dimming” in some Carbon stars.  Carbon stars tend to shed a lot of (“sooty”) material when they expand (Minima) that later shows up when some of them becoming Planetary Nebula. So far in my reading, I have not found any papers that describe this  “dimming behavior” in great detail with the exception of a paper on R For. 

Could it be that the material that is thrown off of the star when it expands is not uniform in thickness but is clumpy?  Or perhaps some changes in the star’s behavior causes this to happen?  It would be good to have as much “color” information as possible when these changes occur and also see if there is change in the spectrum of the star - particularly since the B-V index of the star seems to change so much.  Is S Aur part of a binary system?  Is there a better way to map or get further information about the dimming feature of this star?

In Smirnova’s paper concerning what might be the behavior seen in S Aur and some other Carbon stars he states: “In some carbon stars, the pulsational or small-amplitude irregular variations are superposed by prominent random light obscuration episodes or long-term trends occurring on longer time-scales (e.g. Feast et al. 1984; Whitelock et al. 1997; Alksnis 2003). The obscuration events are observed for roughly one-third of carbon-rich Mira variables and an unknown fraction of SR and Lb variables (Whitelock et al. 2006).”

This same general type of pattern shows up in other Carbon stars, with some of them having cycles of “brightening” and some with periods of “dimming”.   These stars include R CMi, S Cep, V CrB, RV Cyg, V Cyg, R For, V Hya, R Lep, RU Vir, CT Lac, R Scl, SS Vir and LX Cyg and perhaps more.   This seems to be a “feature” that shows up in mostly Carbon stars.  In the “Further Information” section below, there are lists of at least 100 Carbon stars, most of which I haven’t looked at.  It might be fun to look at more of these and develop an AAVSO LPV webpage tracking more of these Carbon stars!

Continual Visual observations and CCD images (B, V, Rc, and Ic bands) need to be continued to help understand the difference in the light coming from the star during the “bright” cycles and that coming from the "dim" cycles.  This star needs to be followed closely for the next 20+ years!

Further information

Carbon star lists

There are a number of lists of Carbon stars that can be used for a target reference:

Observing Carbon stars

Carbon stars are so red that they can cause special problems for Visual observers.

Sky & Telescope has a very good article talking about observing Carbon stars at:

https://www.skyandtelescope.com/observing/carbon-stars-will-make-see-red...

Purkinje effect

Use this link to get a good definition and description of the Purkinje effect that can make accurate Visual observations difficult: https://en.wikipedia.org/wiki/Purkinje_effect

This is the biggest problem for Visual observers with dealing with very dark red LPVs.  The AAVSO has an excellent forum discussion about dealing with this effect at:

https://www.aavso.org/determining-carbon-star-magnitude-direct-v-averted

References and further reading.

Dusek, J., Mikulasek, Z., Papousek, J. “Preliminary analysis of light curves of seven carbon stars”, 2003, Contrib. Astron. Obs. Skalnate Plesco, 33, 119-133

Feast, M. W., et al. “Variable circumstellar obscuration of the carbon star R Fornacis”, 1984,  Monthly Notices of the Royal Astronomical Society, 211, 331-337

Groenewegen, M.A.T., van den Hoek, L.B., de Jong, T. “The evolution of galactic carbon stars”, 1995, Astronomy and Astrophysics, 293, 381

Knapp, G. R., et al. “The light curve and evolutionary status of the carbon star V Hya”, 1999, Astronomy and Astrophysics ***

Mikulasek, Z., Graf, T. “Atlas of H alpha emission lines and V light curves of 30 carbon Miras”, 2005, Contrib. Astron. Obs. Skalnate Plesco, 35, 83-106

Richichi, A., Chandrasekhar, T. “Near-infrared observations of the carbon stars TU Geminorum and S Aurginis at millisecond resolution”, 2006, Astronomy and Astrophysics, 451, 1041-1044

Smirnova, O. “High-resolution spectroscopy of two carbon stars with long-term obscuration events”, 2012, Monthly Notices of the Royal Astronomical Society, 424, 2468-2476

Whitelock, P. A., et al. “Mass-loss variations amoung carbon-rich AGB variables”, 1997,  Monthly Notices of the Royal Astronomical Society, 288, 512-532

Whitelock, P. A., et al. “Near-infrared photometry of carbon stars”, 2006, Monthly Notices of the Royal Astronomical Society, 369, 751-782

 

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