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Variable Star Of The Month
November, 2002: VW Hydri
VW Hydri: Tops in the Dwarf Nova
Superlatives  | | An artist's impression of a cataclysmic variable (CV) star. For a review of this
type of variable, see one of our many CV
features, including the prototype of the SU UMa class.
Image credit: Mark A. Garlick |
When one thinks
of the dwarf nova subclass of cataclysmic variables, a number of familiar names come to mind: SS Cyg, U Gem, SU UMa, and Z Cam.
To this list, VW Hyi should be no exception. While residing at a declination of -71°, VW Hyi is one of the most popular dwarf novae in the southern skies. More
specifically, VW Hyi is one of the most favored SU UMa-type dwarf novae. The SU UMa class of dwarf novae is
well known for their normal and superoutburst behavior. That is, in addition to the normal dwarf nova
outburst, which consists of a rise from quiescence of 2-6 magnitudes and 1-3 day durations, the SU UMa
variables also displays bouts of superoutbursts. As the name suggests, such outbursts are longer and
brighter than the normal outbursts, though they occur less frequently. Superoutbursts may occur every 3-10
cycles, last for 10-18 days, and may rise in brightness by at least an additional magnitude. The light
curves of such variables are also punctuated with an additional feauture whereby small-amplitude periodic
modulations (variations) of several tenths of a magnitude are detected. This phenomenon, known as the
superhump, is seen to occur with a period that is 2-3% longer than the period of the system. Therefore, by
observing the superhumps, one can obtain the orbital period of the system. SU UMa-type stars have been found
(in almost all cases) to have orbital periods of less than 2 hours! Spotlight on VW Hydri  |  | | A portion of the VW Hyi light curve from the AAVSO International Database. The top panel shows
the myriad of normal and superoutbursts that VW Hyi goes through in a course of several years. The bottom
panel shows a closer view of these outbursts. Click on either image for an enlarged view.
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While we now know generally about the SU UMa stars, what can we specifically say
about VW Hyi? Numerous studies have been carried out to analyze as many properties of this popular star as
possible. According to these studies (see Hack and la Dous 1993 and references therein) we know that the
average time between two normal outbursts is 27.3 days, while for superoutbursts it is 179 days, and that
the outbursts last for 1.4 and 12.6 days, respectively. During quiescence, VW Hyi can be seen around
magnitude 14.4. The outbursts, however, can be seen around 8.4 for the superoutburst around 9.0 or so for
the normal outburst. According to Smak (1985), the amplitude and width of a normal outburst are related to
the length of the preceding cycle, but not the following cycle. In all, between 3 to 8 normal outbursts
occur between two consecutive supercycles--the name given to the time between two consecutive
superoutbursts. In fact, two types of supercycles are observed: a longer one, which is strong with the last
two cycles having a duration of more than 60 days and a shorter one, where the outbursts are weak and last
two cycles are less than 45 days (Smak 1985; Hack and la Dous 1993). The star then rises to and reaches
superoutburst and the superhump appears when maximum light is achieved. Through 5-color photometry it has
been found that the shape of the superhump is relatively the same with all filters, but that the amplitude
of the superhump decreases with shorter wavelengths. Studies have shown that amplitudes in all colors
decrease with time as the superoutburst progresses (van Amerongen et al. 1987; Hack and la Dous 1993). At
the other extreme, during quiescence, VW Hyi shows a periodically repeating hump that reveals an orbital
period of 0.07427111 days or 107 minutes (Vogt 1974)! Congratulations on Collaborations  | | In addition to the HST, Astro-1 and -2 missions, VW Hyi has also been
observed by an alphabet soup of satellites: IUE, HEAO-2, EXOSAT, ROSAT, GINGA, ASCA, and EUVE. The HST, pictured above, is probably the
most famous of the bunch. |
Professional astronomers have benefited greatly from
the patience and enthusiasm of amateur astronomers time and time again. Due to time or financial
restrictions, professional astronomers simply cannot carry out long-term, dedicated studies of objects that
the amateurs can. In turn, they've looked to organizations such as the AAVSO for help from its hundreds of
observers worldwide. Whether it is a correlation between data, or a trigger event where the astronomers need
to know when a particular star will be in a specific state, amateur astronomers have heeded the call
faultlessly. VW Hyi has been a component in the proud partnership that exists between these astronomers with
a common goal: to further our understanding of variable stars. For instance, studies of CVs, such as VW
Hyi, in quiescence with the Hubble Space Telescope (HST) gave astronomers the opportunity to explore the
interaction between accreting disk gas and the white dwarf surface layers. According to Sion (1996) and
references therein, models predict the following parameters for white dwarf in the VW Hyi system: a minimum
age since formation of ~ 50 million years; an effective photospheric temperature of 22,000 ± 2000 K;
and a rotational velocity of V sin i » 600 km/s. In addition to
the parameters given above, studies with the HST also revealed: "(a) the first rotational velocities of for
the underlying white dwarfs in cataclysmic variables; (b) the first white dwarf masses independent of disk
emission lines; (c) the first chemical abundances for accreted atmospheres; (d) the first definitive
evidence of white dwarf cooling in response to the dwarf nova accretion heating process in several systems"
(Sion 1996). On another front, again through collaborations between professional astronomers and amateur
astronomers, researchers working with the Hopkins Ultraviolet Telescope (HUT) on Astro-1 and Astro-2 were
able to monitor several dwarf novae--some through outburst, some through quiescence. The result yielded the
first set of far ultraviolet spectra that provided good emission and absorption lines (Long 1995). From a
researcher's point of view, obtaining such data is important as it can be used to put constraints on models
of dwarf novae. More recently, by combining the visual light curves of the AAVSO and VSS/RASNZ and the ultraviolet light curves from the EUV, Mauche et al.
(2001) found the time delay between the optical and ultraviolet components of light curves for several dwarf
novae, including VW Hyi. Knowing this behavior, again, gives astronomers more clues about the nature of
dwarf nova outbursts. Through continued amateur support, professional astronomers will be able to piece
more and more of the puzzle together to unlock the secrets of variable stars! How Can I Observe VW Hydri?  | | The constellation of Hydrus was first published in Johann Bayer's Uranometria atlas.
Bayer's Uranometria opened a new age in the history of celestial cartography, and was praised for the
careful placement of star positions and brightnesses and for its attractive plates. Click on the above image
for an enlarged view. Image credit: U.S. Naval Observatory
Library |
Since the constellation of Hydrus (not to be confused with the
constellation of Hydra) is unviewable from latitudes north of +32°,
observers at southern latitudes and those near the equator are best suited for observing the dwarf nova VW
Hyi. The variable itself can be found just over 3° northeast of the 3rd
magnitude star gamma Hyi. Brightness estimates may then be made by using the 'b' and 'd' scale AAVSO
charts. Those interested in adding VW Hyi to an observing program should plan to observe the variable
every clear night, if possible. Superoutbursts should be monitored every 5 minutes for at least a 2-3 hour
period (on a given night) for evidence of superhumps. Although such activity is best detected by equipment
sensitive to small-amplitude variations, such as photoelectric photometers and CCDs, the visual observer
might also try a hand at detecting the fluctuations. Observations of VW Hyi may then be submitted to the AAVSO for addition to the AAVSO International
Database. VW Hyi has been in the AAVSO observing program since 1958. In the past 40+ years, AAVSO
observers have logged over 13,500 observations for this variable. The light curve from 1961 onward for this
and other variables may be viewed by using the Light Curve Generator. If
you are curious about the present activity of VW Hyi, visit the Quick
Look file, or scan the most recent News Flashes for
up-to-date information and reports on upcoming amateur-professional collaborations. - AAVSO 'b' & 'd' scales
for 0409-71 VW Hydri
- AAVSO VSOTM for February
2000: SU Ursae Majoris
- Hack, M. and C. la Dous, eds.
Cataclysmic Variables and Related Objects. Washington, DC: NASA Scientific and Technical Information
Branch, 1993.
- Long, K.S. "The Hopkins
Ultraviolet Telescope Observations of Dwarf Novae." Journal of the AAVSO, 23, 1995, 94-105.
- Mauche, C.W. has web page where he
gives an excellent review of cataclysmic variables.
- Mauche, C.W., J. Mattei, and F. Bateson.
"Optical and EUV Light Curves of Dwarf Novae Outbursts." In P. Podsiadlowski, S. Rappaport, A.R. King, F.
D'Antona, and L. Burder, eds. ASP Conference Series, Volume 229: Evolution of Binary and Multiple Star
Systmes; A Meeting in Celebration of Peter Eggleton's 60th Birthday. San Franciso: ASP, 2001,
367-371.
- Sion, E.M. "Hubble Space
Telescope Studies of Exposed White Dwarfs in Dwarf Novae." Journal of the AAVSO, 24, 1996,
1-8.
- Smak, J. "Statistical Analysis
of Outbursts and Superoutbursts of VW Hydri." Acta Astronomica, 35, 1985, 357-367.
- van
Amerongen, S., H. Bovenschen, and J. van Paradijs. "Wavelength
Dependence of Superhumps in VW Hydri." Monthly Notices of the Royal Astronomical Society, 229,
1987, 245-251.
- Vogt, N. "Photometric Study
of the Dwarf Nova VW Hydri." Astronomy & Astrophysics, 36, 1974, 369-378.
This
month's Variable Star of the Month was prepared by Kerri Malatesta, AAVSO Technical
Assistant.
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