ACTIVITY ON THE M STAR OF QS Vir (original) (raw)
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HS 2231+2441: an HW Vir system composed by a low-mass white dwarf and a brown dwarf
2017
HW Vir systems are rare evolved eclipsing binaries composed by a hot compact star and a low-mass main-sequence star in a close orbit. These systems provide a direct way to measure the fundamental properties, e.g. masses and radii, of their components, hence they are crucial to study the formation of sdB stars and low-mass white dwarfs, the common-envelope phase, and the pre-phase of cataclysmic variables. Here we present a detailed study of HS 2231+2441, an HW Vir type system, by analysing BVR_CI_C photometry and phase-resolved optical spectroscopy. The spectra of this system, which are dominated by the primary component features, were fitted using NLTE models providing effective temperature Teff = 28500±500 K, surface gravity log g = 5.40±0.05 cm s^-2, and helium abundance log(n(He)/n(H)) = -2.52±0.07. Geometrical orbit and physical parameters were derived by modelling simultaneously the photometric and spectroscopic data using the Wilson-Devinney code. We derive two possible solut...
An active K0 IV--V star and a hot white dwarf (EUVE J0702+129) in a wide binary
Astronomy and Astrophysics
We present far ultraviolet and optical spectroscopy of the Extreme Ultraviolet Explorer (EUVE) survey source EUVE J0702+129 revealing a composite K0 star plus DA white dwarf spectrum. The International Ultraviolet Explorer spectra show continuum emission from a hot white dwarf (T eff = 30 − 40, 000 K) and a rising contribution from the K0 star at λ ≥ 2500Å. High resolution optical spectroscopy uncovers a high level of activity with strong Hα and Ca H&K emission; application of the Wilson-Bappu relation indicates that the secondary star is slightly above the main sequence (K0 IV-V). Both objects are found at a distance of ∼ 130 pc and they likely constitute a physical pair. The EUV emission is dominated by the white dwarf, but the late-type star certainly contributes at higher energy. An interesting parallel is drawn with other DA+K0 pairs with moderately active secondaries such as HD 18131 and HR 1608. The present discovery as well as other recent ones demonstrate the existence of a large population of white dwarfs hidden by evolved companions (III-IV).
HS 2231+2441: an HW Vir system composed of a low-mass white dwarf and a brown dwarf
HW Vir systems are rare evolved eclipsing binaries composed of a hot compact star and a low-mass main sequence star in a close orbit. These systems provide a direct way to measure the fundamental properties, e.g. masses and radii, of their components, hence they are crucial in studying the formation of subdwarf B stars and low-mass white dwarfs, the common-envelope phase and the pre-phase of cataclysmic variables. Here, we present a detailed study of HS 2231+2441, an HW Vir type system, by analysing BVR C I C photometry and phase-resolved optical spectroscopy. The spectra of this system, which are dominated by the primary component features, were fitted using non-local thermodynamic equilibrium models providing an effective temperature T eff = 28 500 ± 500 K, surface gravity log g = 5.40 ± 0.05 cm s −2 and helium abundance log (n(He)/n(H)) = −2.52 ± 0.07. The geometrical orbit and physical parameters were derived by simultaneously modelling the photometric and spectroscopic data using the Wilson–Devinney code. We derive two possible solutions for HS 2231+2441 that provide the component masses: M 1 = 0.19 M and M 2 = 0.036 M or M 1 = 0.288 M and M 2 = 0.046 M. Considering the possible evolutionary channels for forming a compact hot star, the primary of HS 2231+2441 probably evolved through the red-giant branch scenario and does not have a helium-burning core, which is consistent with a low-mass white dwarf. Both solutions are consistent with a brown dwarf as the secondary.
Observations and Orbital Analysis of the Giant White Dwarf Binary System HR 5692
The Astronomical Journal, 2011
We report spectroscopic observations of the red giant star HR 5692, previously known to be a binary system both from other spectroscopic work and from deviations in the astrometric motion detected by the Hipparcos satellite. Earlier International Ultraviolet Explorer (IUE) observations had shown the presence of a hot white dwarf companion to the giant primary. We have combined our radial velocity observations with other existing measurements and with the Hipparcos intermediate astrometric data to determine a complete astrometric-spectroscopic orbital solution, providing the inclination angle for the first time. We also determine an improved parallax for the system of 10.12 ± 0.67 mas. We derive the physical properties of the primary, and with an estimate of its mass from stellar evolution models (1.84±0.40 M), we infer the mass of the white dwarf companion to be M WD = 0.59 ± 0.12 M. An analysis of an IUE white dwarf spectrum, using our parallax, yields T eff = 30,400 ± 780 K, log g = 8.25 ± 0.15, and a mass M WD = 0.79 ± 0.09 M , in marginal agreement with the dynamical mass.
AB Dor in '94: I. HST/GHRS Observations of the Quiescent Chromosphere of an Active Star
We analyze HST/GHRS spectra of AB Doradus, the prototypical ultra-rapidly rotating K dwarf. We observed chromospheric (Mg II) and transition region (C II, Si IV, C IV, and N V) lines periodically throughout the stellar rotation period, and provide a low dispersion stellar atlas of 78 emission lines. The quiescent line profiles of the chromospheric and transition region lines show narrow cores superposed on very broad wings. The broad wings of the Mg II k & h lines and of the transition region lines can be explained by emission from gas co-rotating with the star and extending out to near the Keplerian co-rotation radius (2.8 stellar radii). While this is not a unique solution, it is consistent with previous studies of Hα emission that are naturally explained by large co-rotating prominences. We find no evidence for rotational modulation of the emission line fluxes. The density diagnostics suggest that the transition region is formed at constant pressure, with an electron density 2-3 × 10 12 cm −3 at a temperature of 3×10 4 K. The electron pressure is about 100 times larger than that for the quiet Sun. The emission measure distribution shows a minimum between log(T) = 5 and 5.5. The Mg II 1 GHRS Investigation Definition Team -4line exhibits three interstellar absorption components along the 15 pc line of sight. We identify the lowest velocity component with the G cloud, but the other components are not identified with any interstellar clouds previously detected from other lines of sight.
The nature of the close magnetic white dwarf + probable brown dwarf binary SDSSJ121209.31+013627.7
Monthly Notices of The Royal Astronomical Society, 2006
Optical time series photometry of the short period magnetic white dwarf + probable brown dwarf binary SDSS 121209.31+013627.7 reveals pulse-like variability in all bands from i' to u', peaking at u'. These modulations are most likely due to a self-eclipsing accretion hot spot on the white dwarf, rotating into view every 88.43 minutes. This period is commensurate with the radial velocity period determined by Schmidt et al. 2005 of ~90 minutes, and consistent with the rotation period of the accretor being equal to the binary orbital period. We combine our observations with those recently published by Koen and Maxted 2006 to provide an accurate ephemeris. We also detect the system in X-rays with Swift, and estimate the accretion rate at ~1x10^-13Msun per year. We suggest that SDSS1212 is most likely a magnetic cataclysmic variable in an extended state of very low accretion, similar to the well-studied Polar EF Eri. Alternatively, the putative brown dwarf is not filling its Roche Lobe and the system is a detached binary in which the white dwarf is efficiently accreting from the wind of the secondary. Six such post-common envelope, ``pre-Polar'' systems - termed ``low accretion rate Polars (LARPs)'' by Schwope et al. 2002 - have previously been identified through optical cyclotron emission lines. Cyclotron emission from SDSS1212 has recently been detected in the near-IR Debes et al. 2006 but, if detached, it would be the first ``LARP'' with a probably sub-stellar secondary. It is unclear whether an L-dwarf wind is strong enough to provide the measured accretion rate. We suggest further observations to distinguish between the Roche Lobe over-flow and wind accretion scenarios.
Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. I. DENIS J220002. 05− 303832.9 AB
The Astronomical Journal, 2006
We present the discovery of the common proper motion M9 + L0 binary DE-NIS J220002.05−303832.9AB, identified serendipitously with the SpeX near infrared imager/spectrograph. Spectral types are derived from resolved near infrared spectroscopy of the wellseparated (1. ′′ 09±0. ′′ 06) components and comparison to equivalent data for M and L dwarf spectral standards. Physical association is deduced from the angular proximity of the sources, their common proper motion and their similar spectrophotometric distances (35±2 pc). The estimated distance of this pair implies a projected separation of 38±3 AU, wider than typical separations for other M dwarf/L dwarf binaries, but consistent with the maximum separation/total system mass trend previously identified by . We discuss the DENIS 2200−3038AB system in context with other low mass binaries, and its role in studying dust formation processes and activity trends across the transition between the M and L dwarf spectral classes.
MOST Photometry and DDO Spectroscopy of the Eclipsing (White Dwarf + Red Dwarf) Binary V471 Tau
The Astronomical Journal, 2007
The Hyades K2V+WD system 471 Tau is a prototype post-common envelope system and a likely cataclysmic binary progenitor. We present 10 days of nearly continuous optical photometry by the MOST (Microvariability & Oscillations of STars) satellite and partly simultaneous optical spectroscopy from DDO (David Dunlap Observatory) of the binary. The photometric data indicate that the spot coverage of the K dwarf component was less than observed in the past, suggesting that we monitored the star close to a minimum in its activity cycle. Despite the low spot activity, we still detected seven flare-like events whose estimated energies are among the highest ever observed in V471 Tau and whose times of occurrence do not correlate with the binary orbital phase. A detailed O − C analysis of the times of eclipse over the last ∼ 35 years reveals timing variations which could be explained in several ways, including perturbations by an as-yet-undetected third body in the system or by a small orbital eccentricity inducing slow apsidal motion. The DDO spectra result in improved determinations of the K dwarf projected rotation velocity, V K sin i = 92 km s −1 , and the orbital amplitude, K K = 150.5 km s −1. The spectra also allow us to measure changes in Hα emission strength and radial velocity (RV) variations. We measure a larger Hα velocity amplitude than found previously suggesting that the source of the emission in V471 Tau was less concentrated around the sub-white-dwarf point on the K star than had been observed in previous studies.
A Spectroscopic Survey of a Sample of Active M Dwarfs
Astronomical Journal, 2002
A moderate resolution spectroscopic survey of Fleming's sample of 54 X-ray selected M dwarfs with photometric distances less than 25 pc is presented. Radial and rotation velocities have been measured by fits to the H-alpha profiles. Radial velocities have been measured by cross correlation. Artificial broadening of an observed spectrum has produced a relationship between H-alpha FWHM and rotation speed, which we use to infer rotation speeds for the entire sample by measurement of the H-alpha emission line. We find 3 ultra-fast rotators (UFRs, vsini > 100km/s), and 8 stars with 30 < vsini < 100 km/s. The UFRs have variable emission. Cross-correlation velocities measured for ultra-fast rotators (UFRs) are shown to depend on rotation speed and the filtering used. The radial velocity dispersion of the sample is 17 km/s. A new double emission line spectroscopic binary with a period of 3.55 days has been discovered, and another known one is in the sample. Three other objects are suspected spectroscopic binaries, and at least six are visual doubles. The only star in the sample observed to have significant lithium is a known TW Hya Association member, TWA 8A. These results show that there are a number of young (< 10^8 yr) and very young (< 10^7 yr) low mass stars in the immediate solar neighbourhood. The H-alpha activity strength does not depend on rotation speed. Our fast rotators are less luminous than similarly fast rotators in the Pleiades. They are either younger than the Pleiades, or gained angular momentum in a different way.
2010
We report the discovery of a faint L6 \pm 1 companion to the previously known M9 dwarf, 2MASS J01303563-4445411, based on our near-infrared imaging and spectroscopic observations with the 3m Infrared Telescope Facility SpeX imager/spectrometer. The visual binary is separated by 3. 28 \pm 0. 05 on the sky at a spectrophotometric distance of 40 \pm 14 pc. The projected physical separation is 130 \pm 50 AU, making it one of the widest VLM field multiples containing a brown dwarf companion. 2MASS J0130-4445 is only one of ten wide VLM pairs and only one of six in the field. The secondary is considerably fainter ({\Delta}K ~ 2.35 mag) and redder ({\Delta} (J - Ks) ~ 0.81 dex), consistent with component near-infrared types of M9.0 \pm 0.5 and L6 \pm 1 based on our resolved spectroscopy. The component types suggest a secondary mass well within the hydrogen-burning limit and an age-dependent mass ratio of 0.6-0.9. The system's space motion and spectroscopic indicators suggest an age of 2-4 Gyr while the model-dependent masses and binding energies suggest that this system is unlikely to have formed via dynamical ejection. The age, composition, and separation of the 2MASS J01303563-4445411 system make it useful for tests of VLM formation theories and of condensate cloud formation in L dwarfs.