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Register a new userDiscovery of a compact companion to the hot subdwarf star BD+37 442
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Added by
Nicola La Palombara
Publication date
2012
fields
Physics
and research's language is
English
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We report the results of the first X-ray observation of the luminous and helium-rich O-type subdwarf BD+37 442, carried out with the XMM-Newton satellite in August 2011. X-ray emission is detected with a flux of about 3x10^(-14) erg/cm2/s (0.2-1 keV) and a very soft spectrum, well fit by the sum of a blackbody with temperature kT_BB = 45^(+11)_(-9) eV and a power law with a poorly constrained photon index. Significant pulsations with a period of 19.2 s are detected, indicating that the X-ray emission originates in a white dwarf or neutron star companion, most likely powered by accretion from the wind of BD+37 442.
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We report on the results we obtained with XMM-Newton observations of HD49798 and BD+37 442, the only two sdO stars for which X-ray emission has been observed so far. HD is a single-lined spectroscopic binary with orbital period of 1.5 days. We could establish that its companion is a massive white dwarf with M = 1.28 Msun, which makes it a candidate type Ia supernova progenitor; we also detected a significant X-ray emission during the white-dwarf eclipse, which could be X-ray emission of the sdO star itself. In the case of BD+37 442, a luminous He-rich sdO that up to now was believed to be a single star, we discovered soft X-ray emission with a periodicity of 19.2 s. This indicates that also this hot subdwarf has a compact binary companion, either a white dwarf or a neutron star, most likely powered by accretion from the wind of the sdO star.
TESS photometry of the extremely helium-rich hot subdwarfs BD+37 442 and BD+37 1977 demonstrates multi-periodic low-amplitude variability with principal periods of 0.56 and 1.14 d, respectively, and with both first and second harmonics present. The lightcurves are not perfectly regular, implying additional periodic and/or non-periodic content. Possible causes are examined, including the binary hypothesis originally introduced to explain X-ray observations, differentially rotating surface inhomogeneities, and pulsations. If the principal photometric periods correspond to the rotation periods, the stars are rotating at approximately 0.7 and 0.3 x breakup, respectively. Surface Rossby waves (r modes) therefore provide the most likely solution.
We report the results of a new textit{XMM-Newton} observation of the helium-rich hot subdwarf BD+37$^{circ}$442 carried out in February 2016. The possible periodicity at 19 s seen in a 2011 shorter observation is not confirmed, thus dismissing the evidence for a binary nature. This implies that the observed soft X-ray emission, with a luminosity of a few 10$^{31}$ ergs$^{-1}$, originates in BD+37$^{circ}$442 itself, rather than in an accreting neutron star companion. The X-ray spectrum is well fit by thermal plasma emission with a temperature of 0.22 keV and non-solar element abundances. Besides the overabundance of He, C and N already known from optical/UV studies, the X-ray spectra indicate also a significant excess of Ne. The soft X-ray spectrum and the ratio of X-ray to bolometric luminosity, L$_{rm X}$/L$_{rm BOL}sim2times10^{-7}$, are similar to those observed in massive early-type stars. This indicates that the mechanisms responsible for plasma shock-heating can work also in the weak stellar winds (mass loss rates $dot M_{rm W} leq10^{-8}$ ${rm M}_{odot}$ yr$^{-1}$) of low-mass hot stars.
We report on the results of the first XMM-Newton satellite observation of the luminous and helium-rich O-type subdwarf BD +37{deg} 1977 carried out in April 2014. X-ray emission is detected with a flux of about 4*10^(-14) erg/cm2/s (0.2-1.5 keV), corresponding to a f_X/f_bol ratio about 10^(-7); the source spectrum is very soft, and is well fit by the sum of two plasma components at different temperatures. Both characteristics are in agreement with what is observed in the main-sequence early-type stars, where the observed X-ray emission is due to turbulence and shocks in the stellar wind. A smaller but still significant stellar wind has been observed also in BD +37{deg} 1977; therefore, we suggest that also in this case the detected X-ray flux has the same origin.
Radial velocity (RV) searches for exoplanets have surveyed many of the nearest and brightest stars for long-term velocity variations indicative of a companion body. Such surveys often detect high-amplitude velocity signatures of objects that lie outside the planetary mass regime, most commonly those of a low-mass star. Such stellar companions are frequently discarded as false-alarms to the main science goals of the survey, but high-resolution imaging techniques can be employed to either directly detect or place significant constraints on the nature of the companion object. Here, we present the discovery of a compact companion to the nearby star HD~118475. Our Anglo-Australian Telescope (AAT) RV data allow the extraction of the full Keplerian orbit of the companion, found to have a minimum mass of 0.445~$M_odot$. Follow-up speckle imaging observations at the predicted time of maximum angular separation rule out a main sequence star as the source of the RV signature at the 3.3$sigma$ significance level, implying that the companion must be a low-luminosity compact object, most likely a white dwarf. We provide an isochrone analysis combined with our data that constrain the possible inclinations of the binary orbit. We discuss the eccentric orbit of the companion in the context of tidal circularization timescales and show that non-circular orbit was likely inherited from the progenitor. Finally, we emphasize the need for utilizing such an observation method to further understand the demographics of white dwarf companions around nearby stars.
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