3,020 research outputs found
UVES and X-Shooter spectroscopy of the emission line AM CVn systems GP Com and V396 Hya
We present time-resolved spectroscopy of the AM CVn-type binaries GP Com and
V396 Hya obtained with VLT/X-Shooter and VLT/UVES. We fully resolve the narrow
central components of the dominant helium lines and determine radial velocity
semi-amplitudes of km s for GP Com and
km s for V396 Hya. The mean velocities of
the narrow central components show variations from line to line. Compared to
calculated line profiles that include Stark broadening we are able to explain
the displacements, and the appearance of forbidden helium lines, by additional
Stark broadening of emission in a helium plasma with an electron density
cm. More than nitrogen and more than
neon lines emission lines were detected in both systems. Additionally,
nitrogen absorption lines are only seen in GP Com. The radial velocity
variations of these lines show the same phase and velocity amplitude as the
central helium emission components. The small semi-amplitude of the central
helium emission component, the consistency of phase and amplitude with the
absorption components in GP Com as well as the measured Stark effect shows that
the central helium emission component, the so-called central-spike, is
consistent with an origin on the accreting white dwarf. We use the dynamics of
the bright spot and the central spike to constrain the binary parameters for
both systems and find a donor mass of - M for GP
Com and - M for V396 Hya. We find an upper limit
for the rotational velocity of the accretor of km s for
GP Com and km s for V396 Hya which excludes a fast
rotating accretor in both systems.Comment: Accepted for publication in MNRAS, 15 pages, 14 figures, 5 table
Hot subdwarf stars in close-up view. I. Rotational properties of subdwarf B stars in close binary systems and nature of their unseen companions
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)The origin of hot subdwarf B stars (sdBs) is still unclear. About half of the known sdBs are in close binary systems for which common envelope ejection is the most likely formation channel. Little is known about this dynamic phase of binary evolution. Since most of the known sdB systems are single-lined spectroscopic binaries, it is difficult to derive masses and unravel the companions' nature, which is the aim of this paper. Due to the tidal influence of the companion in close binary systems, the rotation of the primary becomes synchronised to its orbital motion. In this case it is possible to constrain the mass of the companion, if the primary mass, its projected rotational velocity as well as its surface gravity are known. For the first time we measured the projected rotational velocities of a large sdB binary sample from high resolution spectra. We analysed a sample of 51 sdB stars in close binaries, 40 of which have known orbital parameters comprising half of all such systems known today. Synchronisation in sdB binaries is discussed both from the theoretical and the observational point of view. The masses and the nature of the unseen companions could be constrained in 31 cases. We found orbital synchronisation most likely to be established in binaries with orbital periods shorter than . Only in five cases it was impossible to decide whether the sdB's companion is a white dwarf or an M dwarf. The companions to seven sdBs could be clearly identified as late M stars. One binary may have a brown dwarf companion. The unseen companions of nine sdBs are white dwarfs with typical masses. The mass of one white dwarf companion is very low. In eight cases (including the well known system KPD1930+2752) the companion mass exceeds , four of which even exceed the Chandrasekhar limit indicating that they may be neutron stars. Even stellar mass black holes are possible for the most massive companions. The distribution of the inclinations of the systems with low mass companions appears to be consistent with expectations, whereas a lack of high inclinations becomes obvious for the massive systems. We show that the formation of such systems can be explained with common envelope evolution and present an appropriate formation channel including two phases of unstable mass transfer and one supernova explosion. The sample also contains a candidate post-RGB star, which rotates fast despite its long orbital period. The post-RGB stars are expected to spin-up caused by their ongoing contraction. The age of the sdB is another important factor. If the EHB star is too young, the synchronisation process might not be finished yet. Estimating the ages of the target stars from their positions on the EHB band, we found PG 2345+318, which is known not to be synchronised, to lie near the zero-age extreme horizontal branch as are the massive candidates PG 1232-136, PG 1432+159 and PG 1101+249. These star may possibly be too young to have reached synchronisation. The derived large fraction of putative massive sdB binary systems in low inclination orbits is inconsistent with theoretical predictions. Even if we dismiss three candidates because they may be too young and assume that the other sdB primaries are of low mass, PG 1743+477 and, in particular, HE 0532-4503 remain as candidates whose companions may have masses close to or above the Chandrasekhar limit. X-ray observations and accurate photometry are suggested to clarify their nature. As high inclination systems must also exist, an appropriate survey has already been launched to find such binaries.Peer reviewe
Massive stars in the hinterland of the young cluster, Westerlund 2
Accepted for publication in MNRAS, 13 July 2018. 16 pages, plus one-page table in an appendix.An unsettled question concerning the formation and distribution of massive stars is whether they must be born in massive clusters and, if found in less dense environments, whether they must have migrated there. With the advent of wide-area digital photometric surveys, it is now possible to identify massive stars away from prominent Galactic clusters without bias. In this study we consider 40 candidate OB stars found in the field around the young massive cluster, Westerlund 2, by Mohr-Smith et al.: these are located inside a box of 1.5 × 1.5 deg 2 and are selected on the basis of their extinctions and K magnitudes.We present VLT/X-shooter spectra of two of the hottest O stars, respectively 11 and 22 arcmin from the centre of Westerlund 2. They are confirmed as O4V stars, with stellar masses likely to be in excess of 40 M ·. Their radial velocities relative to the non-binary reference object, MSP 182, in Westerlund 2 are -29.4 ± 1.7 and -14.4 ± 2.2 km s -1, respectively. Using Gaia DR2 proper motions we find that between 8 and 11 early O/WR stars in the studied region (including the two VLT targets, plus WR 20c and WR 20aa) could have been ejected fromWesterlund 2 in the last one million years. This represents an efficiency of massive-star ejection of up to ~ 25 per cent. On sky, the positions of these stars and their proper motions show a near N-S alignment. We discuss the possibility that these results are a consequence of prior sub-cluster merging combining with dynamical ejection.Peer reviewe
Discovery of 36 eclipsing EL CVn binaries found by the Palomar Transient Factory
We report the discovery and analysis of 36 new eclipsing EL CVn-type
binaries, consisting of a core helium-composition pre-white dwarf and an
early-type main-sequence companion, more than doubling the known population of
these systems. We have used supervised machine learning methods to search 0.8
million lightcurves from the Palomar Transient Factory, combined with SDSS,
Pan-STARRS and 2MASS colours. The new systems range in orbital periods from
0.46-3.8 d and in apparent brightness from ~14-16 mag in the PTF or
filters. For twelve of the systems, we obtained radial velocity
curves with the Intermediate Dispersion Spectrograph at the Isaac Newton
Telescope. We modelled the lightcurves, radial velocity curves and spectral
energy distributions to determine the system parameters. The radii (0.3-0.7
) and effective temperatures (8000-17000 K) of the
pre-He-WDs are consistent with stellar evolution models, but the masses
(0.12-0.28 ) show more variance than models predicted. This
study shows that using machine learning techniques on large synoptic survey
data is a powerful way to discover substantial samples of binary systems in
short-lived evolutionary stages
Phase resolved spectroscopy and Kepler photometry of the ultracompact AM CVn binary SDSS J190817.07+394036.4
{\it Kepler} satellite photometry and phase-resolved spectroscopy of the
ultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. The
average spectra reveal a variety of weak metal lines of different species,
including silicon, sulphur and magnesium as well as many lines of nitrogen,
beside the strong absorption lines of neutral helium. The phase-folded spectra
and the Doppler tomograms reveal an S-wave in emission in the core of the He I
4471 \AA\,absorption line at a period of \,sec
identifying this as the orbital period of the system. The Si II, Mg II and the
core of some He I lines show an S-wave in absorption with a phase offset of
compared to the S-wave in emission. The N II, Si III and some
helium lines do not show any phase variability at all. The spectroscopic
orbital period is in excellent agreement with a period at \,sec detected in the three year {\it Kepler} lightcurve. A
Fourier analysis of the Q6 to Q17 short cadence data obtained by {\it Kepler}
revealed a large number of frequencies above the noise level where the majority
shows a large variability in frequency and amplitude. In an O-C analysis we
measured a xs\,s for some of
the strongest variations and set a limit for the orbital period to be
s\,s. The shape of the phase folded
lightcurve on the orbital period indicates the motion of the bright spot.
Models of the system were constructed to see whether the phases of the radial
velocity curves and the lightcurve variation can be combined to a coherent
picture. However, from the measured phases neither the absorption nor the
emission can be explained to originate in the bright spot.Comment: Accepted for publication in MNRAS, 15 pages, 14 figures, 5 table
A radio-pulsing white dwarf binary star
White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic4. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco’s optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10^7-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf’s spin, they mainly originate from the cool star. AR Sco’s broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere
KIC7668647: a 14 day beaming sdB+WD binary with a pulsating subdwarf
The recently discovered subdwarf B (sdB) pulsator KIC7668647 is one of the 18
pulsating sdB stars detected in the Kepler field. It features a rich g-mode
frequency spectrum, with a few low-amplitude p-modes at short periods.
We use new ground-based low-resolution spectroscopy, and the near-continuous
2.88 year Kepler lightcurve, to reveal that KIC7668647 consists of a subdwarf B
star with an unseen white-dwarf companion with an orbital period of 14.2d. An
orbit with a radial-velocity amplitude of 39km/s is consistently determined
from the spectra, from the orbital Doppler beaming seen by Kepler at 163ppm,
and from measuring the orbital light-travel delay of 27 by timing of the many
pulsations seen in the Kepler lightcurve. The white dwarf has a minimum mass of
0.40 M_sun.
We use our high signal-to-noise average spectra to study the atmospheric
parameters of the sdB star, and find that nitrogen and iron have abundances
close to solar values, while helium, carbon, oxygen and silicon are
underabundant relative to the solar mixture.
We use the full Kepler Q06--Q17 lightcurve to extract 132 significant
pulsation frequencies. Period-spacing relations and multiplet splittings allow
us to identify the modal degree L for the majority of the modes. Using the
g-mode multiplet splittings we constrain the internal rotation period at the
base of the envelope to 46-48d as a first seismic result for this star. The few
p-mode splittings may point at a slightly longer rotation period further out in
the envelope of the star.
From mode-visibility considerations we derive that the inclination of the
rotation axis of the sdB in KIC7668647 must be around ~60 degrees.
Furthermore, we find strong evidence for a few multiplets indicative of
degree 3 <= L <= 8, which is another novelty in sdB-star observations made
possible by Kepler.Comment: arXiv admin note: text overlap with arXiv:1206.387
The physical properties of AM CVn stars: new insights from Gaia DR2
AM CVn binaries are hydrogen deficient compact binaries with an orbital
period in the 5-65 min range and are predicted to be strong sources of
persistent gravitational wave radiation. Using Gaia Data Release 2, we present
the parallaxes and proper motions of 41 out of the 56 known systems. Compared
to the parallax determined using the HST Fine Guidance Sensor we find that the
archetype star, AM CVn, is significantly closer than previously thought. This
resolves the high luminosity and mass accretion rate which models had
difficulty in explaining. Using Pan-STARRS1 data we determine the absolute
magnitude of the AM CVn stars. There is some evidence that donor stars have a
higher mass and radius than expected for white dwarfs or that the donors are
not white dwarfs. Using the distances to the known AM CVn stars we find strong
evidence that a large population of AM CVn stars have still to be discovered.
As this value sets the background to the gravitational wave signal of LISA,
this is of wide interest. We determine the mass transfer rate for 15 AM CVn
stars and find that the majority have a rate significantly greater than
expected from standard models. This is further evidence that the donor star has
a greater size than expected.Comment: Accepted by A&A in main journa
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