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A Yellow Supergiant in a Spectroscopic Binary System?

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A&A 570, A3 (2014) Astronomy DOI: 10.1051/0004-6361/201322888 & c ESO 2014 Astrophysics HD 50975: a yellow supergiant in a spectroscopic binary system? J. Sperauskas1, L. Zacsˇ 1;2, S. Raudeliunas¯ 1, F. Musaev3;4;5, and V. Puzin3 1 Vilnius University Observatory, Ciurlionio 29, Vilnius 03100, Lithuania e-mail: [email protected] 2 Laser Center, University of Latvia, Rain¸a bulvaris¯ 19, 1586 R¯ıga, Latvia 3 Institute of Astronomy of the Russian AS, 48 Pyatnitskaya st., 119017 Moscow, Russia 4 Terskol Branch of Institute of Astronomy of the Russian AS, 361605 Peak Terskol, Kabardino-Balkaria, Russia 5 Special Astrophysical Observatory of the Russian AS, 369167 Nizhnij Arkhyz, Russia Received 21 October 2013 / Accepted 4 August 2014 ABSTRACT Context. Recent detection of a yellow supergiant star as a possible progenitor of a supernova has posed serious questions about our understanding of the evolution of massive stars. Aims. The spectroscopic binary star HD 50975 with an unseen hot secondary was studied in detail with the main goal of estimating fundamental parameters of both components and the binary system. Methods. A comprehensive analysis and modeling of collected long-term radial velocity measurements, photometric data, and spectra was performed to calculate orbital elements, atmospheric parameters, abundances, and luminosities. The spectrum in an ultraviolet region was studied to clarify the nature of an unseen companion star. Results. The orbital period was found to be 190:22±0:01 days. The primary star (hereafter HD 50975A) is a yellow supergiant with an effective temperature Teff = 5900 ± 150 K and a surface gravity of log(g) = 1:4 ± 0:3 (cgs). The atmosphere of HD 50975 A is slightly metal deficient relative to solar, [Fe=H] = −0:26 ± 0:06 dex. Abundances of Si and Ca are close to the scaled solar composition. The r-process element europium is enhanced, [Eu=H] = +0:61 ± 0:07. The bolometric magnitude of the primary was estimated to be Mbol = −5:5 ± 0:3 mag and its mass to be 10:7 ± 2:0 M . The secondary (hereafter HD 50975B) is a hot star of spectral type ∼B2 near ZAMS with an effective temperature of Teff ' 21 000 K and a mass M ' 8:6 M . The distance between HD 50975A and B is about 370 R : The binary star is near a semi-detached configuration with a radius, RA ' 107 R , and a radius of Roche lobe of about 120 R for the primary star. Key words. binaries: spectroscopic – stars: evolution – stars: fundamental parameters – stars: individual: HD 50975 – stars: abundances 1. Introduction satellite TD1 and noted that HD 50975 is an “unexplained case”. HD 50975 is located in the region of G4 subdwarfs according Recent detection of yellow supergiant star as the possible pro- to uvby photometry (Olsen 1979), and low-amplitude light vari- genitor of supernovae have raised serious questions about our ations are suspected by Olsen (1983). The period (P) and am- understanding of the evolution of massive stars. Bersten et al. plitude (A) of light variations were determined by Koen & Eyer (2012) and Benvenuto et al. (2013) present recent evidence (2002), P = 95:1 days and A = 0:013 mag. Radial velocity varia- that the yellow supergiant star found at the location of super- tions were detected for HD 50975 by Bartkevicius & Sperauskas nova SN 2011dh in the famous nearby galaxy M51 was the (1994). SN progenitor. The authors of these papers have produced a In the present paper a comprehensive analysis and modeling self-consistent model to explain how the star exploded. In their of the collected long-term radial velocity measurements, photo- model, the exploding massive star was a member of a close bi- metric data, spectral energy distribution (SED), cross-correlation nary system. A comprehensive analysis of similar binary sys- function (CCF), and high-resolution spectra was performed for tems will advance our knowledge of binary systems as possible HD 50975 to calculate orbital elements, atmospheric parameters, progenitors of supernova explosions. abundances, luminosities, and masses. A spectrum in the ultravi- The spectrum of HD 50975 was classified as a composite olet and blue wavelength region was analyzed in detail to clarify spectrum of two stars, F8 Ib supergiant and an early type star, the nature of unseen companion. using the objective prism with dispersion of 108 Å mm−1 at Hγ by MacConnell & Bidelman (1976). The same spectral type 2. Observations was found for a primary star by Barbier et al. (1978) on the basis of spectra with dispersion of 200 Å mm−1. Barbier et al. Radial-velocity monitoring of HD 50975 was started in 1989 us- (1978) analyzed photometric data gathered using the ultraviolet ing the CORAVEL spectrometer of the Vilnius University in- stalled on the 1.65 m and 0.63 m telescopes at the Moletai ? The reduced spectra are only available at the CDS via anonymous Observatory (Lithuania). The CORAVEL spectrometer (Upgren ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via et al. 2002) is based on the principles of the photoelectric radial- http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A3 velocity scanner developed by Griffin (1967) and operates by Article published by EDP Sciences A3, page 1 of9 A&A 570, A3 (2014) scanning a spectrum of star across the mask and obtaining an on- Table 1. Radial velocity measurements for HD 50975A, along with the line cross-correlation velocity. The standard deviation of a single uncertainties, phases, and residuals for the orbital curve. observation for late-type stars brighter than about the 11th mag- −1 nitude is usually better than 0.8 km s . The velocities have been HJD-2 400 000 Epoch Phase Vr (O–C) σ standardized using the observations of IAU radial velocity stan- days km s−1 km s−1 km s−1 dards (Udry et al. 1999) and are close to the system of velocities 47 572.171 37 .598260 0.30 −1.161 0.6 published by Nidever et al. (2002). The difference in zero point 47 575.173 37 .614042 3.40 −0.602 0.5 was found to be 0.14 km s−1 for F-G-K type stars and exhibit an 47 924.292 35 .449374 −3.10 1.008 0.6 rms scatter of 0.5 km s−1. 47 935.288 35 .507181 −5.10 1.044 0.6 High-resolution spectra for HD 50975 were obtained with 47 942.190 35 .543465 −5.60 −0.949 0.6 the coudé échelle spectrometer MAESTRO on the 2 m telescope 47 950.213 35 .585642 0.50 0.832 0.5 − − at the Observatory on the Terskol Peak in Northern Caucasus (al- 48 302.251 33 .436320 3.60 0.686 0.5 48 305.242 33 .452044 −3.40 0.920 0.7 titude of 3100 m) equipped with a Wright Instruments CCD de- 48 315.201 33 .504399 −6.70 −0.530 0.6 tector. Three spectra were obtained between November 2012 and 48 319.283 33 .525858 −4.00 1.641 1.2 January 2013. In addition, spectra of several hot, rapidly rotat- 48 321.260 33 .536251 −5.60 −0.484 0.8 ing stars were observed to identify the telluric absorption lines. 48 334.238 33 .604477 2.80 0.376 0.5 All the spectra covered region from about 3600 to 9600 Å and 48 347.220 33 .672724 16.80 0.893 0.5 have a resolving power R ∼ 45 000. Unfortunately, our efforts to 48 349.228 33 .683280 18.20 −0.178 0.6 observe a high-resolution spectrum shortward of 3600 Å was un- 48 351.208 33 .693689 20.80 −0.087 0.6 48 656.284 31 .297486 23.30 0.237 0.6 successful. All spectra were bias-subtracted, flat-field-corrected, 48 658.228 31 .307706 20.90 0.353 0.9 and converted to one-dimensional spectra using the standard − 1 48 660.281 31 .318498 17.90 0.057 0.8 DECH20T package . The equivalent widths of absorption lines 48 663.278 31 .334254 14.70 0.380 0.8 were measured by Gaussian fitting to the observed profiles. 48 665.213 31 .344426 12.30 0.221 1.0 48 666.198 31 .349604 12.10 1.126 0.6 48 670.200 31 .370643 6.60 −0.160 0.6 3. Analysis and results 48 672.196 31 .381136 4.10 −0.740 0.6 48 676.272 31 .402564 −0.20 −1.538 0.8 3.1. Orbital solution and light curve 49 038.222 29 .305349 19.60 −1.522 0.7 Orbital parameters for HD 50975 were calculated using 53 ra- 49 042.276 29 .326661 16.40 0.351 0.7 dial velocities observed in the time span from February 1989 49 046.189 29 .347232 11.90 0.423 0.6 49 050.206 29 .368350 5.20 −1.997 0.7 to April 2008. The measured heliocentric radial velocities and 49 058.201 29 .410380 0.00 −0.210 0.7 their errors are presented in Table1, together with the time 49 061.262 29 .426472 −1.90 −0.057 0.7 of observations, phases, and residuals (O–C). Orbital elements 49 065.239 29 .447379 −3.70 0.242 0.5 are listed in Table2. All of the radial velocities upon which 51 268.292 17 .028917 76.40 0.414 0.7 the orbit is based have been attributed the same (unit) weight. 51 269.271 17 .034064 75.90 0.177 0.9 Radial velocity curve for HD 50975A is shown in Fig.1 (upper 51 271.289 17 .044673 75.20 0.153 0.7 panel).
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