arXiv subjects

Péter Németh

Publications and source records attributed to Péter Németh.

22 records · Page 2Linked to original sources

arXiv2017

Composite hot-subdwarf binaries -- I. The spectroscopically confirmed sdB sample

Hot subdwarf-B (sdB) stars in long-period binaries are found to be on eccentric orbits, even though current binary-evolution theory predicts these objects to be circularised before the onset of Roche-lobe overflow (RLOF). To increase our understanding of binary interaction processes during the RLOF phase, we started a long term observing campaign to study wide sdB binaries. In this article we present a composite-binary-sdB sample, and the results of the spectral analysis of 9 such systems. The grid search in stellar parameters (GSSP) code is used to derive atmospheric parameters for the cool companions. To cross-check our results and also characterize the hot subdwarfs we used the independent XTgrid code, which employs Tlusty non-local thermodynamic equilibrium models to derive atmospheric parameters for the sdB component and Phoenix synthetic spectra for the cool companions. The independent GSSP and XTgrid codes are found to show good agreement for three test systems that have atmospheric parameters available in the literature. Based on the rotational velocity of the companions, an estimate for the mass accreted during the RLOF phase and the miminum duration of that phase is made. It is found that the mass transfer to the companion is minimal during the subdwarf formation.

astro-ph.SR↗

arXiv2016

An extremely fast halo hot subdwarf star in a wide binary system

New spectroscopic observations of the halo hyper-velocity star candidate SDSS J121150.27+143716.2 ($V=17.92$ mag) revealed a cool companion to the hot subdwarf primary. The components have a very similar radial velocity and their absolute luminosities are consistent with the same distance, confirming the physical nature of the binary, which is the first double-lined hyper-velocity candidate. Our spectral decomposition of the Keck/ESI spectrum provided an sdB+K3V pair, analogous to many long-period subdwarf binaries observed in the Galactic disk. We found the subdwarf atmospheric parameters: $T_{\rm eff}=30\,600\pm500$ K, $\log{g}=5.57\pm0.06$ cm s$^{-2}$ and He abundance $\log(n{\rm He}/n{\rm H})=-3.0\pm0.2$. Oxygen is the most abundant metal in the hot subdwarf atmosphere, and Mg and Na lines are the most prominent spectral features of the cool companion, consistent with a metallicity of $[{\rm Fe}/{\rm H}]=-1.3$. The non-detection of radial velocity variations suggest the orbital period to be a few hundred days, in agreement with similar binaries observed in the disk. Using the SDSS-III flux calibrated spectrum we measured the distance to the system $d=5.5\pm0.5$ kpc, which is consistent with ultraviolet, optical, and infrared photometric constraints derived from binary spectral energy distributions. Our kinematic study shows that the Galactic rest-frame velocity of the system is so high that an unbound orbit cannot be ruled out. On the other hand, a bound orbit requires a massive dark matter halo. We conclude that the binary either formed in the halo or it was accreted from the tidal debris of a dwarf galaxy by the Milky Way.

astro-ph.SR↗

arXiv2013

Non-LTE Luminosity and Abundance Diagnostics of Classical Novae in X-rays

Classical novae are significant sources of interstellar material, especially carbon, nitrogen and oxygen. These standard candles are only behind supernovae and $γ$-ray bursts as the third brightest objects in the sky, and the most probable progenitors of type Ia supernovae. After a nova outburst the system enters into the constant bolometric luminosity phase and the nova maintains a stable hydrogen burning in the surface layers of the white dwarf. As the expanding shell around the nova attenuates, progressively deeper and hotter layers become visible. At the end of the constant bolometric luminosity phase, the hottest layers are exposed and novae radiate X-rays. This work uses the static, plane-parallel model atmosphere code TLUSTY to calculate atmospheric structure and SYNSPEC to calculate synthetic X-ray spectra. It was necessary to incorporate atomic data for the highest ionization stages of elements ranging from hydrogen to iron in both programs. Atomic data on energy levels, bound-free, bound-bound transitions and natural broadening were taken from NIST and TOPbase. Extensive tests revealed the importance of line opacities on atmospheric parameters and on the final spectra. A correlation can be defined between effective temperature and surface gravity. The spectral appearance is not very sensitive to the joint changes of both. Due to this effect both parameters might be over-estimated with static models. These tests also showed that N VI and N VII lines are good indicators of the effective temperature. Model fitting of V4743 Sgr and V2491 Cyg confirmed the anticipated impact of modeling geometry and stellar wind. The ionization balance and line profiles indicate that both novae are close to or over the Eddington limit. These results are consistent with previous studies; further and unambiguous details require a comprehensive update of TLUSTY, what is under way.

astro-ph.SR↗

arXiv2012

A selection of hot subluminous stars in the GALEX survey II. Subdwarf atmospheric parameters

We present an update of our low-resolution spectroscopic follow-up and model atmosphere analysis of hot subdwarf stars from the Galaxy Evolution Explorer (GALEX) survey. Targets were selected on the basis of colour indices calculated from the GALEX GR6 N_UV, Guide Star Catalogue (GSC2.3.2) V and the Two Micron All Sky Survey (2MASS) J and H photometry. High signal-to-noise ratio spectra were obtained at the European Southern Observatory (ESO) and the Kitt Peak National Observatory (KPNO) over the course of three years. Detailed H, He and CNO abundance analysis helped us improve our T_eff, log g and He abundance determination and to constrain CNO abundances. We processed 191 observations of 180 targets and found 124 sdB and 42 sdO stars in this sample while some blue horizontal branch stars were also found in this programme. With quantitative binary decomposition of 29 composite spectra we investigated the incidence of A, F and G type companions. The incidence of late G and K type companions and their effects on subdwarf atmospheric parameters were also examined.

astro-ph.SR↗