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A New Extreme Helium Star Identified Using SALT

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A New Extreme Helium Star Identified Using SALT Mon. Not. R. Astron. Soc. 000, 1–7 (2014) Printed 13 June 2017 (MN LATEX style file v2.2) GALEX J184559.8−413827: a new extreme helium star identified using SALT⋆ C. Simon Jeffery1,2† 1Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK 2School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland Accepted . Received . ; in original form . ABSTRACT A high-resolution spectrum of the helium-rich ‘hot subdwarf’ GALEX J184559.8−413827 (J1845−4138) obtained with SALT HRS demonstrates it to be the first extreme helium (EHe) star to be discovered in nearly 40 years. A quantitative analysis demonstrates it to have an atmosphere described by Teff = 26170 ± 750 K, log g/cms−2 = 4.22 ± 0.10, and a surface chemistry characterised by CNO-processed helium, a 1% contamination of hydrogen (by number), and a metallicity 0.4 dex subsolar. Its distance and position are consistent with membership of the Galactic bulge. Its sharp absorption lines place strong constraints on both the rotation and microturbulent velocities. Spectroscopically, J1845−4138 closely resembles the pulsating EHe star V652 Her, generally considered to be the product of a double helium white dwarf merger evolving to become a helium-rich sdO star. Key words: stars: abundances, stars: fundamental parameters, stars: chemically peculiar, stars: individual (GALEX J184559.8−413827, V652 Her) 1 INTRODUCTION veys of luminous blue stars complete to B = 12 for the Milky Way and for b = ±30◦ and l = ±60◦ (Drilling & Bergeron Extreme helium stars (EHes) comprise some seventeen stars 1995; Drilling 1996), fainter EHes would either lie well out- of spectral types equivalent to A and B but having weak side the Milky Way or be heavily obscured and reddened by or no hydrogen Balmer lines. In their place, relatively sharp its central bulge. At the least luminous end of their range, and strong lines of neutral helium indicate low surface grav- EHe stars have surface gravities similar to those seen on the ities and atmospheres dominated by helium. The next most main-sequence. The EHes appear to be substantially less abundant elements are carbon, nitrogen and oxygen, in- arXiv:1706.03377v1 [astro-ph.SR] 11 Jun 2017 numerous. dicative of highly processed material exposed at the stel- lar surface. The first extreme helium star, HD124448, was Since all of the classical EHes were discovered, there identified by Popper (1942). This was followed by dis- have been numerous spectroscopic surveys for faint blue coveries spanning some thirty years, many arising from stars (sometimes masquerading as quasar or galaxy sur- spectroscopic surveys of luminous blue stars. All of these veys), including the Palomar-Green (Green et al. 1986), have been analysed spectroscopically in more or less de- Hamburg quasar and Hamburg/ESO (Hagen et al. 1995; tail, giving effective temperature and surface gravities; a Wisotzki et al. 1996), Edinburgh-Cape (Stobie et al. 1997), full list is provided by Jeffery (2008), to which should HK (Beers et al. 1992), Sloan (York et al. 2000) and be added analyses of BD+10◦2179 by Pandey & Lambert GALEX (Bianchi et al. 2014) surveys. Each adopted a (2011) and Kupfer et al. (2017). Translating surface grav- scheme to classify the spectra of stellar objects and thereby ity into luminosity-to-mass ratio (L/M), the class covers a identified many new and interesting stars. Drilling (1996) range of nearly 2 dex in L/M, with the brightest members recognized that the class identified by Green et al. (1986) i lying close to the Eddington limit. Hence, the fact that there as sdOD (“pure He absorption spectra, characterized by have been no new luminous EHes discovered since about the weakness or absence of hydrogen Balmer lines and He ii i 1980 (Drilling & Hill 1986) is easily understood. With sur- 4686 while showing the singlet He 4388 about equal in strength to the triplet He i 4471”), and by Moehler et al. (1990) as He-sdB, is the same definition as that given for extreme helium stars and binaries by Drilling & Hill (1986). ⋆ based on an observation made with the Southern African Large Yet no new EHe stars were found. Telescope (SALT) † email: [email protected] The sdOD/He-sdB classification also embraces helium- 2 C. S. Jeffery rich stars of higher surface gravity, i.e. helium-rich hot sub- a median seeing of 1.3′′. Two exposures were obtained in dwarfs. Considerable effort has been spent on discovering each of the HRS cameras, the first with an exposure time of and exploring these stars, with spectacular results. They 1300 s, the second was terminated after 700 s by a technical turn out to fall into diverse groups, including the dou- problem. A second observation was attempted the following ble subdwarf PG1544+488 (Schulz et al. 1991; Ahmad et al. night, but the wrong star was observed. 2004; S¸ener & Jeffery 2014), the binary CPD-20◦1123 The HRS spectrum was reduced to 1-dimension (to- (Naslim et al. 2012), through the chemically-peculiar in- tal counts versus wavelength) object and sky orders using termediate helium subdwarfs (Naslim et al. 2011, 2013; the SALT HRS pipeline2 (Crawford et al. 2016). Flux cal- Jeffery et al. 2017), the low-gravity He-sdO stars and O(He) ibration was not attempted. The wavelength ranges cov- stars (Husfeld et al. 1989; Reindl et al. 2014) to the high- ered by the spectra are 3860 – 5519 A˚ and 5686 – 8711 gravity extreme helium sdO stars (Greenstein & Sargent A˚ respectively, the combined spectrum having a S/N ra- 1974; Dreizler et al. 1990; Str¨oer et al. 2007; Naslim et al. tio in the range 20 – 30 at a resolution of ≈ 37, 000. The 2010; Justham et al. 2011; Zhang & Jeffery 2012). Drilling orders were rectified, mapped onto a common wavelength (1996) and Drilling et al. (2013) introduced an MK-like grid (equally spaced in log wavelength) and merged using classification scheme which clearly distinguishes between an order-management tool written for ´echelle spectra by the helium-rich subdwarfs and extreme helium stars, but re- author. Rectification at low exposure levels remains prob- quires a resolution closer to 1.5A˚ than the 10A˚ of many lematic, in the current case leading to problems at the blue surveys. Hence, survey spectra have rarely been classified end of the blue spectrum. using the Drilling scheme1. Stars of interest to us have more Fig. 1 shows that the He i absorption lines are readily frequently been classified as He-sdB, He-sdOB, or He-sdO apparent and everywhere stronger than the Balmer lines. Efforts to find more examples of some subclasses require He ii 4686A,˚ can be identified, but not the He ii Pickering high-resolution and high signal-to-noise spectra to identify series. The He i lines are everywhere narrower than seen in weak and double lines. A spectroscopic survey of stars clas- helium-rich hot subdwarfs with log g ≥ 5, but less sharp sified as helium-rich subdwarfs, especially He-sdB or sdOD than the lowest-gravity extreme helium stars. On the other using 8m class telescopes has been undertaken. This paper hand the spectrum is remarkably similar to that of the bet- reports an observation of one such star obtained with the ter known EHe star V652 Her (Jeffery et al. 2015), including high-resolution spectrograph (HRS) on the Southern Africa in particular the extremely rich spectrum of singly-ionized Large Telescope (SALT) (Buckley et al. 2006; Bramall et al. nitrogen (Fig. 1). The most marked difference is that He ii 2010; Crause et al. 2014) 4686A˚ is significantly stronger in J1845−4138, indicating a higher Teff . A detailed analysis to determine the surface properties more precisely is presented in the next section. A single value for the heliocentric radial velocity of 2 OBSERVATIONS J1845−4138 was measured by comparing the observed ◦ ′ ′′ spectrum obtained at 2017 03 17 02h 24m (Julian Date GALEX J184559.8−413827 (α2000 = 18 45 59.8 , δ2000 = ◦ ′ ′′ − 2457828.6000) with the best-fit theoretical spectrum, yield- 41 38 27 , V = 14.6: J1845 4138 hereafter ) was iden- −1 tified to be a faint blue star from GALEX colours by ing a value −66.1 ± 0.2kms . There was no discernible Vennes et al. (2011). A flux-calibrated low-resolution spec- shift between the two exposures. The mean value is ap- trum obtained with the Faint Object Spectrograph and proximately consistent with a series of five measurements made over the space of three nights in 2012 and which gave Camera mounted on the European Southern Observatory’s −1 New Technology Telescope (EFOSC2/NTT) on 2008 Octo- v¯ = −57.6 ± 6.1kms (Kawka et al. 2015). ber 21 was classified ‘He-sdB’, being dominated by neutral helium lines. A coarse analysis of the spectrum and colours yielded first an effective temperature Teff = 36 400±3 200 K, 3 ANALYSIS surface gravity log g/cm s−1 = 5.75 ± 0.65, and surface A grid of model atmospheres and theoretical spectra helium-to-hydrogen ratio > 0.40 (Vennes et al. 2011), and +840 −1 +0.27 was prepared based on the approximate composition for second Teff = 35 930−4770 K, log g/cm s = 5.75−0.23 , +1.10 V652 Her (Jeffery et al. 2001). The input composition for log nHe/nH = 2.10 (N´emeth et al. 2012). Both stud- −0.38 the model grid was checked for consistency with the mea- ies used the same observations; the first used atmospheres sured abundances, and an iteration was carried out where comprising hydrogen and helium only, but in which depar- necessary. The observed spectrum used to fit Teff and g tures from local thermodynamic equilibrium were considered was the order-merged spectrum direct from the rectifica- (non-LTE) , while the second used non-LTE models compris- tion procedure.
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