30 research outputs found
Observational asteroseismology of hot subdwarf stars
Hot subdwarf stars are particularly challenging for asteroseismology due to
their rapid pulsation periods, intrinsic faintness and relative rarity both in
the field and in clusters. These features have ensured that the preferred
method of observation up to now has been white-light photometry, and all
asteroseismological solutions to date have been made by model fitting of the
frequency spectrum. Several attempts have been made to perform asteroseismology
using time-resolved spectroscopy on the brightest of these stars, but with
modest results. A few attempts at simultaneous multi-color photometry have also
been made to identify modes with the amplitude ratio method. We will review the
most recent observational results and progress in improving the observational
methods for ground-based asteroseismology of these compact pulsators.Comment: 8 pages, to appear in Astronomische Nachrichten, Vol. 331, 102
Discovery of two bright DO-type white dwarfs
We discovered two bright DO-type white dwarfs, GALEXJ053628.3+544854
(J0536+5448) and GALEX231128.0+292935(J2311+2929), which rank among the eight
brightest DO-type white dwarfs known. Our non-LTE model atmosphere analysis
reveals effective temperatures and surface gravities of
and
for J0536+5448 and and for J2311+2929. The latter shows a significant amount
of carbon in its atmosphere (, by mass), while for
J0536+5448 we could derive only an upper limit of . Furthermore, we
calculated spectroscopic distances for the two stars and found a good agreement
with the distances derived from the Gaia parallaxes.Comment: 7 pages, 4 figures, accepted for publication in MNRA
Looking at the bright side - The story of AA Dor as revealed by its cool companion
Irradiation effects in close binaries are crucial for a reliable
determination of system parameters and understanding the close binary
evolution. We study irradiated light originating from the low mass component of
an eclipsing system comprising a hot subdwarf primary and a low mass companion,
to precisely interpret their high precision photometric and spectroscopic data,
and accurately determine their system and surface parameters. We re-analyse the
archival VLT/UVES spectra of AA Dor system where irradiation features have
already been detected. After removing the predominant contribution of the hot
subdwarf primary, the residual spectra reveal more than 100 emission lines from
the heated side of the secondary with maximum intensity close to the phases
around secondary eclipse. We analyse 22 narrow emission lines of the irradiated
secondary, mainly of OII, with a few CII lines. Their phase profiles constrain
the emission region of the heated side to a radius 95% of the radius of
the secondary. The shape of their velocity profiles reveals two distinct
asymmetry features one at the quadrature and the other at the secondary
eclipse. We identify more than 70 weaker emission lines originating from HeI,
NII, SiIII, CaII and MgII. We correct the radial velocity semi-amplitude of the
center-of-light to the centre-of-mass of the secondary and calculate accurate
masses of both components. The resulting masses =0.46
0.01 and =0.079 0.002 are in perfect
accordance with those of a canonical hot subdwarf primary and a low mass star
just at the substellar limit for the companion. We compute a first generation
atmosphere model of the irradiated low mass secondary, which matches the
observed spectrum well. We find an indication of an extended atmosphere of the
irradiated secondary star.Comment: 13 pages, 9 figures, accepted for publication in A&
A Radial Velocity Study of Composite-Spectra Hot Subdwarf Stars with the Hobby-Eberly Telescope
Many hot subdwarf stars show composite spectral energy distributions
indicative of cool main sequence companions. Binary population synthesis (BPS)
models demonstrate such systems can be formed via Roche lobe overflow or common
envelope evolution but disagree on whether the resulting orbital periods will
be long (years) or short (days). Few studies have been carried out to assess
the orbital parameters of these spectroscopic composite binaries; current
observations suggest the periods are long. To help address this problem, we
selected fifteen moderately-bright (V~13) hot subdwarfs with F-K dwarf
companions and monitored their radial velocities (RVs) from January 2005 to
July 2008 using the bench-mounted Medium Resolution Spectrograph on the
Hobby-Eberly Telescope (HET). Here we describe the details of our observing,
reduction, and analysis techniques and present preliminary results for all
targets. By combining the HET data with recent observations from the Mercator
telescope, we are able to calculate precise orbital solutions for three systems
using more than 6 years of observations. We also present an up-to-date period
histogram for all known hot subdwarf binaries, which suggests those with F-K
main sequence companions tend to have orbital periods on the order of several
years. Such long periods challenge the predictions of conventional BPS models,
although a larger sample is needed for a thorough assessment of the models'
predictive success. Lastly, one of our targets has an eccentric orbit, implying
some composite-spectrum systems might have formerly been hierarchical triple
systems, in which the inner binary merged to create the hot subdwarf.Comment: Published in The Astrophysical Journal, Volume 758, Issue 1, article
id. 58 (2012). References updated and Equation (5) corrected. 12 pages, 5
figures, 5 table
Subaru and salt spectroscopy of chemically peculiar hot subdwarfs
The majority of hot subdwarfs lie on or close to the helium main-sequence. Many have hydrogen-rich surfaces, but a substantial fraction of the hotter subdwarfs have hydrogen-depleted or hydrogen-deficient surfaces. Amongst the former, three were known to show extraordinary overabundances of heavy elements including zirconium and lead. Using Subaru/HDS,we commenced a high-resolution survey of hydrogen-depleted subdwarfs to discover new members of the class. UVO 0825+15, was found to exhibit strong lead lines, to be an intrinsic variable in K2 field 5, and to have a relatively high space motion. Two other lead-rich subdwarfs have been found in the Subaru sample. A much wider survey is in progress using SALT/HRS. Discoveries so far include one extreme helium star similar to V652 Her, and an intermediate helium star with possible comparison to HD144941. Analyses of the hotter and more compact members of the sample are continuing. Open Access
Subaru and SALT spectroscopy of chemically peculiar hot subdwarfs
The majority of hot subdwarfs lie on or close to the helium main-sequence. Many have hydrogen-rich surfaces, but a substantial fraction of the hotter subdwarfs have hydrogen-depleted or hydrogen-deficient surfaces. Amongst the former, three were known to show extraordinary overabundances of heavy elements including zirconium and lead. Using Subaru/HDS,we commenced a high-resolution survey of hydrogen-depleted subdwarfs to discover new members of the class. UVO 0825+15, was found to exhibit strong lead lines, to be an intrinsic variable in K2 field 5, and to have a relatively high space motion. Two other lead-rich subdwarfs have been found in the Subaru sample. A much wider survey is in progress using SALT/HRS. Discoveries so far include one extreme helium star similar to V652 Her, and an intermediate helium star with possible comparison to HD144941. Analyses of the hotter and more compact members of the sample are continuing