PX Andromedae

PX Andromedae (often abbreviated to PX And) is an eclipsing cataclysmic variable star in the constellation Andromeda. It has been classified as a SW Sextantis variable,[3] and its apparent visual magnitude varies between 14.04 and 17.[2]

PX Andromedae
Observation data
Epoch J2000      Equinox J2000
Constellation Andromeda
Right ascension  00h 30m 05.8084s[1]
Declination +26° 17′ 26.4389″[1]
Apparent magnitude (V) 14.04 – 17.00 variable[2]
Characteristics
Spectral type pec(UG)[2]
Apparent magnitude (B) 11.84[3]
Apparent magnitude (R) 14.69[3]
Apparent magnitude (G) 14.8494[1]
Apparent magnitude (J) 14.652[4]
Apparent magnitude (H) 14.485[4]
Apparent magnitude (K) 14.344[4]
Variable type SW Sex[3]
Astrometry
Proper motion (μ) RA: −10.924±0.081 [1] mas/yr
Dec.: −10.992±0.052[1] mas/yr
Parallax (π)1.2323 ± 0.0437[1] mas
Distance2,650 ± 90 ly
(810 ± 30 pc)
Orbit
Period (P)3.5112 hours[5]
Other designations
2MASS J00300581+2617264, AAVSO 0024+25, 2XMM J003005.7+261727
Database references
SIMBADdata

Spectrum

The spectrum of PX Andromedae is variable, but typically shows a continuum with prominent broad emission lines of hydrogen and helium. Unlike many types of cataclysmic variable, the emission lines are generally single-peaked, although for a short time during each orbit they do show a double peak due to an absorption core within the emission lines. The single-peaked lines are shown in novae and dwarf novae outbursts, and PX Andromedae is often described as nova-like although it does not shown outbursts with large increases in brightness. However, it does have high and low states with the low states generally being fainter and showing weaker emission.[6]

PX Andromedae is a very blue star with an ultraviolet excess, indicating that it includes very hot objects.[6]

Variability

PX Andromedae generally has a visual magnitude around 15 although there are continual rapid variations of up to a tenth of a magnitude on a timescale of minutes. Approximately every 3.5 hours the brightness drops by around a magnitude and then returns to the normal brightness in about half an hour, without a flat bottom, easily identifiable as partial eclipses. However, the depth of the eclipses varies from about 0.5 magnitudes to 1.5 magnitudes, and this appears to cycle every 4.8 days, coinciding with the negative superhump cycle. Deeper eclipses occur when the superhump cycle is near its minimum.[7] Possible variations with a period of 0.207 days are likely to be an observational alias of the superhump period. PX Andromedae may have both high (brighter) and low (fainter) states for its mean brightness, but the timing of these is uncertain.[5]

The eclipses have been timed to occur every 0.1463 days, assumed to be the orbital period. The negative superhump period is 4.43 days. Positive superhumps have sometimes been claimed, but are not always present.[5]

Stars with this type of variation are classified as SW Sextantis variables and PX Andromedae is sometimes considered as one of the prototypes for the class.[8] Although they are considered to be cataclysmic variables and are often described as "nova-like", they do not show intermittent outbursts. Rather they are in a continuous outburst state with a spectrum similar to an ongoing nova.[9]

System

Since the spectral classification of PX Andromedae is peculiar and similar to U Geminorum,[2] it's commonly accepted that in this system a white dwarf is accreting matter from a donor star, and an accretion disc has formed around the former. The negative superhumps show that the accretion disk is tilted with respect to the white dwarf rotation axis, and has a retrogade precession. Moreover, the donor star doesn't eclipse the central part of the disk, otherwise there wouldn't be any modulation of the eclipse depth.[5]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. N. N. Samus; O. V. Durlevich; et al. "PX And database entry". Combined General Catalog of Variable Stars (2017 ed.). CDS. Retrieved 2018-11-12.
  3. Norton, A. J.; Wheatley, P. J.; West, R. G.; Haswell, C. A.; Street, R. A.; Collier Cameron, A.; Christian, D. J.; Clarkson, W. I.; Enoch, B.; Gallaway, M.; Hellier, C.; Horne, K.; Irwin, J.; Kane, S. R.; Lister, T. A.; Nicholas, J. P.; Parley, N.; Pollacco, D.; Ryans, R.; Skillen, I.; Wilson, D. M. (2007). "New periodic variable stars coincident with ROSAT sources discovered using SuperWASP". Astronomy and Astrophysics. 467 (2): 785. arXiv:astro-ph/0702631. Bibcode:2007A&A...467..785N. doi:10.1051/0004-6361:20077084.
  4. Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; et al. (June 2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues (2246): II/246. Bibcode:2003yCat.2246....0C.
  5. Thomas, N. L. (2010). "SuperWASP observations of long timescale photometric variations in cataclysmic variables". Astronomy and Astrophysics. 514: A30. arXiv:1003.4996. Bibcode:2010A&A...514A..30T. doi:10.1051/0004-6361/200912650.
  6. Still, M. D.; Dhillon, V. S.; Jones, D. H. P. (1995). "Emission-line variations of the nova-like variable PX Andromedae (=PG0027+260)". Monthly Notices of the Royal Astronomical Society. 273 (4): 863. Bibcode:1995MNRAS.273..863S. doi:10.1093/mnras/273.4.863.
  7. Stanishev, V.; Kraicheva, Z.; Boffin, H. M. J.; Genkov, V. (2002). "PX Andromedae: Superhumps and variable eclipse depth". Astronomy and Astrophysics. 394 (2): 625–632. arXiv:astro-ph/0208270. Bibcode:2002A&A...394..625S. doi:10.1051/0004-6361:20021163.
  8. Boyd, D. (2012). "A Study of the Orbital Periods of Deeply Eclipsing SW Sextantis Stars". Journal of the American Association of Variable Star Observers. 40 (1): 295. arXiv:1207.2432. Bibcode:2012JAVSO..40..295B.
  9. Hellier, Coel; Robinson, E. L. (1994). "PX Andromedae and the SW Sexantis Phenomenon". The Astrophysical Journal. 431: L107. Bibcode:1994ApJ...431L.107H. doi:10.1086/187484.
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