DY Pegasi

DY Pegasi, abbreviated DY Peg, is a binary star[5] system in the northern constellation of Pegasus. It is a well-studied[11] SX Phoenicis variable star with a brightness that ranges from an apparent visual magnitude of 9.95 down to 10.62 with a period of 1.75 hours.[3] This system is much too faint to be seen with the naked eye, but can be viewed with large binoculars or a telescope.[12] Based on its high space motion and low abundances of heavier elements, it is a population II star system.[13]

DY Pegasi

A visual band light curve DY Pegasi, plotted from ASAS-SN data[1]
Observation data
Epoch J2000      Equinox J2000
Constellation Pegasus
Right ascension 23h 08m 51.186s[2]
Declination +17° 12′ 56.00″[2]
Apparent magnitude (V) 9.95 – 10.62[3]
Characteristics
Spectral type A3 to F1[4]
Variable type SX Phe[5][3]
Astrometry
Radial velocity (Rv)−25.30±2.7[6] km/s
Proper motion (μ) RA: 47.248 mas/yr[2]
Dec.: −22.103 mas/yr[2]
Parallax (π)2.4588 ± 0.0452 mas[2]
Distance1,330 ± 20 ly
(407 ± 7 pc)
Absolute magnitude (MV)2.34[7]
0.84[8]
Orbit[5]
Period (P)15,425.0±205.7 d
Semi-major axis (a)≥ 0.254±0.034 AU
Eccentricity (e)0.65 ± 0.10
Periastron epoch (T)2438276.86149 ± 0.00013 HJD
Details
Mass1.54 M☉[7]
1.40[8] M☉
Radius2.09±0.25 R☉[4]
3.74 – 3.95[8] R☉
Luminosity11.34+2.82
−2.51 L☉[4]
34.6±2.1[8] L☉
Temperature7,660 K[7] (7,950 – 6,750)[9] K
Metallicity [Fe/H]−0.56[5] dex
Rotational velocity (v sin i)23.6[5] km/s
Age1.7[7] Gyr
Other designations
DY Peg, BD+16°4877, HD 218549, HIP 114290[10]
Database references
SIMBADdata

Observation history

The variability of this star was first reported by Otto Morgenroth in 1934,[5] and the first light curves of its photometric behavior were constructed by A. V. Soloviev in 1938.[14] This curve showed a rapid increase of 0.7 in magnitude followed by a slower decline.[15] It was found to be an intrinsic variable with an "ultra-short" period of 105 minutes. The 'b-v' color index of the star was found to vary with each cycle, corresponding to a change in spectral type from A7 at maximum to F1 at minimum. Direct observation of spectra showed a variation from A3 to A9.[16] Evidence was found of small variations in the light curve between each cycle.[17]

By 1972, it was widely regarded as a dwarf cepheid;[18] a Delta Scuti variable. However, some astronomers classed it as a short-period RRs Lyrae variable.[19] Photometric observations of DY Peg in 1975 by E. H. Geyer and M. Hoffman showed non-periodic changes to the light curve that suggested an overtone pulsation.[20] A frequency analysis of observations made by A. Masani and P. Broglia in 1953 strengthened the evidence that DY Peg is a double mode cepheid, showing a fundamental pulsation and a weaker first overtone with a period ratio of 0.764.[17] By 1982, similarities with SX Phoenicis had been found, with both showing comparable drifts in their beat periods.[21] Application of the Baade-Wesselink method provided a preliminary distance estimate to DY Peg of 820 ly (250 pc).[9]

In 2003, J. N. Fu and C. Sterken suggested that much of the long-term trend in variability period changes could be explained by a highly-eccentric orbital model, although it was not deemed a complete solution since some small residuals remained from the period 1930–1950. They computed a preliminary orbital period of 52.5±0.3 years with an eccentricity of 0.77±0.01.[22] L.-J. Li and S.-B. Qian in 2010 found a mass estimate of the secondary in the range of 0.028 to 0.173 M☉, which suggests the companion may be a brown dwarf.[14]

Properties

A 2020 analysis of data collected by the AAVSO found three independent frequencies in the variability of the visible component. The primary and secondary modes are radial pulsations with 13.71249 and 17.7000 cycles per day, respectively, while a newly discovered non-radial mode has a frequency of 18.138 cycles per day. Consistent with being a population II star, it has a low metallicity.[5] The stellar class ranges from A3 to F1 over each cycle,[4] and the radius of the star varies by 3.5%.[4] To explain certain discrepant properties of the system, H.-F. Xue and J.-S. Niu proposed that the primary may be accreting mass from an orbiting dust disk. This is conjectured to be leftover material from a white dwarf companion as it passed through the asymptotic giant branch.[5]

DY Pegasi has been classified as a SX Phoenicis variable on the basis of its low metallicity. However, a 2014 study by S. Barcza and J. M. Benkő found a much higher general abundance of heavy elements with [M/H] = −0.05±0.1 dex, approaching solar in composition. (This notation indicates the base-10 logarithm of the ratio of "metals" 'M' to hydrogen 'H', compared to the same abundances in the Sun. A value of 0.0 is solar.) They proposed that this may instead be a high amplitude Delta Scuti variable. The short period of this variable rules it out as an RR Lyrae variable.[8]

The properties of DY Pegasi are uncertain due to the presence of an unknown companion, but it appears to lie close to the main sequence at the red (cool) edge of the instability strip.[9] However, it has also been treated as a possible RR Lyrae variable which would be a horizontal branch star.[8] As an old low-metallicity SX Phoenicis variable, it is very similar to blue stragglers, which are formed from stellar mergers or mass transfer in binary systems.[9]

References
  1. "ASAS-SN Variable Stars Database". ASAS-SN Variable Stars Database. ASAS-SN. Retrieved 6 January 2022.
  2. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
  3. Samus', N. N; et al. (2017), "General catalogue of variable stars: Version GCVS 5.1", Astronomy Reports, 61 (1): 80, Bibcode:2017ARep...61...80S, doi:10.1134/S1063772917010085, S2CID 125853869.
  4. Wilson, W. J. F.; et al. (April 1998), "Studies of Large-Amplitude delta Scuti Variables. III. DY Pegasi", The Publications of the Astronomical Society of the Pacific, 110 (746): 433–450, Bibcode:1998PASP..110..433W, doi:10.1086/316148.
  5. Xue, Hui-Fang; Niu, Jia-Shu (November 2020), "DY Pegasi: An SX Phoenicis Star in a Binary System with an Evolved Companion", The Astrophysical Journal, 904 (1): 12, arXiv:2008.02542, Bibcode:2020ApJ...904....5X, doi:10.3847/1538-4357/abbc12, S2CID 221006143, 5.
  6. Gontcharov, G. A. (November 2006), "Pulkovo Compilation of Radial Velocities for 35495 Hipparcos stars in a common system", Astronomy Letters, 32 (11): 759–771, arXiv:1606.08053, Bibcode:2006AstL...32..759G, doi:10.1134/S1063773706110065, S2CID 119231169.
  7. Hintz, Eric G.; et al. (June 2004), "Period Changes in the SX Phoenicis Star DY Pegasi", The Publications of the Astronomical Society of the Pacific, 116 (820): 543–553, Bibcode:2004PASP..116..543H, doi:10.1086/420858, S2CID 120366195.
  8. Barcza, S.; Benkő, J. M. (August 2014), "Fundamental parameters of RR Lyrae stars from multicolour photometry and Kurucz atmospheric models - III. SW And, DH Peg, CU Com and DY Peg", Monthly Notices of the Royal Astronomical Society, 442 (2): 1863–1876, arXiv:1405.4184, Bibcode:2014MNRAS.442.1863B, doi:10.1093/mnras/stu978.
  9. Meylan, G.; et al. (April 1986), "RR Lyrae, delta Scuti, SX Phoenicis stars and Baade-Wesselink method. I. Photometric and radial velocity measurements of four field stars: RR Cet, DX Del, BS AQR and DY Peg", Astronomy and Astrophysics, 159: 261–268, Bibcode:1986A&A...159..261B.
  10. "DY Peg". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2020-01-02.
  11. Fu, J. N.; et al. (March 2009), "Pulsations and Period Changes of the SX Phoenicis Star DY Pegasi", Publications of the Astronomical Society of the Pacific, 121 (877): 251, Bibcode:2009PASP..121..251F, doi:10.1086/597829.
  12. "The astronomical magnitude scale", International Comet Quarterly, retrieved 2020-12-31.
  13. Frolov, M. S.; Irkaev, B. N. (January 1984), "On the SX Phe-Type Stars", Information Bulletin on Variable Stars, 2462: 1, Bibcode:1984IBVS.2462....1F.
  14. Li, L. -J.; Qian, S. -B. (June 2010), "A Period Investigation of the SX Phoenicis Star DY Pegasi", The Astronomical Journal, 139 (6): 2639–2642, Bibcode:2010AJ....139.2639L, doi:10.1088/0004-6256/139/6/2639.
  15. Iriarte, Braulio (September 1952), "A Photoelectric Light-Curve of DY Peg", Astrophysical Journal, 116: 382, Bibcode:1952ApJ...116..382I, doi:10.1086/145621.
  16. Geilker, Chas. D. (July 1957), "Three-color photometry of the ultrashort-period variable DY Pegasi.", Astronomical Journal, 62: 143, Bibcode:1957AJ.....62..143G, doi:10.1086/107494.
  17. Kozar, T. (August 1980), "DY Pegasi - a Double Mode Dwarf Cepheid?", Information Bulletin on Variable Stars, 1834: 1, Bibcode:1980IBVS.1834....1K.
  18. Warner, B.; Nather, R. E. (1972), "Three-colour photometry of DY Pegasi", Monthly Notices of the Royal Astronomical Society, 156 (3): 315, Bibcode:1972MNRAS.156..315W, doi:10.1093/mnras/156.3.315.
  19. Geyer, E. H.; Hoffmann, M. (October 1974), "Maxima of the RRs-variables CY Aqr, DY Her and DY Peg", Information Bulletin on Variable Stars, 936: 1, Bibcode:1974IBVS..936....1G.
  20. Geyer, E. H.; Hoffmann, M. (August 1975), "A two-colour photometry of the short period RR Lyrae star DY Peg.", Astronomy & Astrophysics Supplement Series, 21: 183–188, Bibcode:1975A&AS...21..183G.
  21. Coates, D. W.; et al. (April 1982), "The rates of change of the fundamental and overtone periods of SX Phe", Monthly Notices of the Royal Astronomical Society, 199: 135–139, Bibcode:1982MNRAS.199..135C, doi:10.1093/mnras/199.1.135.
  22. Fu, J. N.; Sterken, C. (July 2003), "Long-term variability of the SX Phoenicis star CY Aquarii", Astronomy and Astrophysics, 405 (2): 685–688, Bibcode:2003A&A...405..685F, doi:10.1051/0004-6361:20030589.
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