51 research outputs found
The CARMENES search for exoplanets around M dwarfs - Photospheric parameters of target stars from high-resolution spectroscopy
The new CARMENES instrument comprises two high-resolution and high-stability
spectrographs that are used to search for habitable planets around M dwarfs in
the visible and near-infrared regime via the Doppler technique. Characterising
our target sample is important for constraining the physical properties of any
planetary systems that are detected. The aim of this paper is to determine the
fundamental stellar parameters of the CARMENES M-dwarf target sample from
high-resolution spectra observed with CARMENES. We also include several M-dwarf
spectra observed with other high-resolution spectrographs, that is CAFE, FEROS,
and HRS, for completeness. We used a {chi}^2 method to derive the stellar
parameters effective temperature T_eff, surface gravity log g, and metallicity
[Fe/H] of the target stars by fitting the most recent version of the
PHOENIX-ACES models to high-resolution spectroscopic data. These stellar
atmosphere models incorporate a new equation of state to describe spectral
features of low-temperature stellar atmospheres. Since T_eff, log g, and [Fe/H]
show degeneracies, the surface gravity is determined independently using
stellar evolutionary models. We derive the stellar parameters for a total of
300 stars. The fits achieve very good agreement between the PHOENIX models and
observed spectra. We estimate that our method provides parameters with
uncertainties of {sigma} T_eff = 51 K, {sigma} log g = 0.07, and {sigma} [Fe/H]
= 0.16, and show that atmosphere models for low-mass stars have significantly
improved in the last years. Our work also provides an independent test of the
new PHOENIX-ACES models, and a comparison for other methods using
low-resolution spectra. In particular, our effective temperatures agree well
with literature values, while metallicities determined with our method exhibit
a larger spread when compared to literature results
The Coronae of AR Lac
We observed the coronally active eclipsing binary, AR Lac, with the High
Energy Transmission Grating on Chandra for a total of 97 ks, spaced over five
orbits, at quadratures and conjunctions. Contemporaneous and simultaneous EUV
spectra and photometry were also obtained with the Extreme Ultraviolet
Explorer. Significant variability in both X-ray and EUV fluxes were observed,
dominated by at least one X-ray flare and one EUV flare. We saw no evidence of
primary or secondary eclipses. X-ray flux modulation was largest at high
temperature, indicative of flare heating of coronal plasma. Line widths
interpreted in terms of Doppler broadening suggest that both binary stellar
components are active. From line fluxes obtained from total integrated spectra,
we have modeled the emission measure and abundance distributions. A strong
maximum was found in the differential emission measure, characterized by peaks
at log T = 6.9 and 7.4, together with a weak but significant cooler maximum
near log T=6.2, and a moderately strong hot tail from log T= 7.6-8.2. Coronal
abundances have a broad distribution and show no simple correlation with first
ionization potential. While the resulting model spectrum generally agrees very
well with the observed spectrum, there are some significant discrepancies,
especially among the many Fe L-lines. Both the emission measure and abundance
distributions are qualitatively similar to prior determinations from other
X-ray and ultraviolet spectra, indicating some long-term stability in the
overall coronal structure.Comment: 31 pages, 8 figures, 3 tables; Accepted for publication in the
Astrophysical Journal (tentatively October 1, 2003
RVSPY -- Radial Velocity Survey for Planets around Young Stars. Target characterization and high-cadence survey
We introduce our Radial Velocity Survey for Planets around Young stars
(RVSPY), characterise our target stars, and search for substellar companions at
orbital separations smaller than a few au from the host star. We use the FEROS
spectrograph to obtain high signal-to-noise spectra and time series of precise
radial velocities (RVs) of 111 stars most of which are surrounded by debris
discs. Our target stars have spectral types between early F and late K, a
median age of 400 Myr, and a median distance of 45 pc. We determine for all
target stars their basic stellar parameters and present the results of the
high-cadence RV survey and activity characterization. We achieve a median
single-measurement RV precision of 6 m/s and derive the short-term intrinsic RV
scatter of our targets (median 22 m/s), which is mostly caused by stellar
activity and decays with age from >100 m/s at 500 Myr.
We discover six previously unknown close companions with orbital periods
between 10 and 100 days, three of which are low-mass stars, and three are in
the brown dwarf mass regime. We detect no hot companion with an orbital period
<10 days down to a median mass limit of ~1 M_Jup for stars younger than 500
Myr, which is still compatible with the established occurrence rate of such
companions around main-sequence stars. We find significant RV periodicities
between 1.3 and 4.5 days for 14 stars, which are, however, all caused by
rotational modulation due to starspots. We also analyse the TESS photometric
time series data and find significant periodicities for most of the stars. For
11 stars, the photometric periods are also clearly detected in the RV data. We
also derive stellar rotation periods ranging from 1 to 10 days for 91 stars,
mostly from TESS data. From the intrinsic activity-related short-term RV
jitter, we derive the expected mass-detection thresholds for longer-period
companions.Comment: 24 pages, 14 figures, 4 tables; Accepted for publication in A&
The Active Quiescence of HR Del (Nova Del 1967)
This new UV study of the ex-nova HR Del is based on all of the data obtained
with the IUE satellite, and includes the important series of spectra taken in
1988 and 1992 that have not been analyzed so far. After the correction for the
reddening (EB-V)=0.16), adopting a distance d =850 pc, we have derived a mean
UV luminosity close to Luv ~ 56 Lsun, the highest value among classical novae
in "quiescence". Also the "average" optical absolute magnitude Mv=+2.30 is
indicative of a bright object. The UV continuum luminosity, the HeII 1640 A
emission line luminosity, and the optical absolute magnitude all give a mass
accretion rate Mdot very close to 1.4*10**(-7) Msun/yr, if one assumes that the
luminosity of the old nova is due to a non-irradiated accretion disk. The
continuum distribution is well fitted with either a black body of 33,900 K, or
a power-law F(lambda) ~ lambda**(-2.20). We show that the "quiescent" optical
magnitude at mv ~ 12 comes from the hot component and not from the companion
star. Since most IUE observations correspond to the "quiescent" magnitude at mv
~ 12, the same as in the pre-eruption stage, we infer that the pre-nova, for at
least 70 years prior to eruption, was also very bright at near the same Luv,
Mv, Mdot and T values as derived in the present study for the ex-nova. The wind
components in the P Cyg profiles of the CIV 1550 A and NV 1240 A resonance
lines are strong and variable on short timescales, with vedge up to -5000 km/s,
a remarkably high value. The phenomenology in the short-time variations of the
wind indicates the presence of an inhomogeneous outflow. We discuss the nature
of the strong UV continuum and wind features and the implications of the
presence of a "bright" state a long time before and after outburst on our
present knowledge of the pre-nova and post-nova behavior.Comment: 15 pages, 10 figures, accepted for Astronomy and Astrophysic
Radial Velocity Survey for Planets around Young Stars (RVSPY). Target characterisation and high-cadence survey
We introduce our Radial Velocity Survey for Planets around Young stars (RVSPY), characterise our target stars, and search for substellar companions at orbital separations smaller than a few au from the host star. We use the FEROS spectrograph to obtain high signal-to-noise spectra and time series of precise radial velocities (RVs) of 111 stars most of which are surrounded by debris discs. Our target stars have spectral types between early F and late K, a median age of 400 Myr, and a median distance of 45 pc. We determine for all target stars their basic stellar parameters and present the results of the high-cadence RV survey and activity characterization. We achieve a median single-measurement RV precision of 6 m/s and derive the short-term intrinsic RV scatter of our targets (median 22 m/s), which is mostly caused by stellar activity and decays with age from >100 m/s at 500 Myr. We discover six previously unknown close companions with orbital periods between 10 and 100 days, three of which are low-mass stars, and three are in the brown dwarf mass regime. We detect no hot companion with an orbital period <10 days down to a median mass limit of ~1 M_Jup for stars younger than 500 Myr, which is still compatible with the established occurrence rate of such companions around main-sequence stars. We find significant RV periodicities between 1.3 and 4.5 days for 14 stars, which are, however, all caused by rotational modulation due to starspots. We also analyse the TESS photometric time series data and find significant periodicities for most of the stars. For 11 stars, the photometric periods are also clearly detected in the RV data. We also derive stellar rotation periods ranging from 1 to 10 days for 91 stars, mostly from TESS data. From the intrinsic activity-related short-term RV jitter, we derive the expected mass-detection thresholds for longer-period companions
Doppler Images and Chromospheric Variability of TWA 6
We present Doppler imaging and Balmer line analysis of the weak-line T Tauri
star TWA 6. Using this data we have made one of the first attempts to measure
differential rotation in a T Tauri star, and the first detection of a slingshot
prominence in such a star. We also show the most direct evidence to date of the
existence of solar-type plages in a star other than the Sun.
Observations were made over six nights: 11-13th February 2006 and 18-20th
February 2006, when spectra were taken with the UCL Echelle Spectrograph on the
3.9-m Anglo-Australian Telescope. Using least-squares deconvolution to improve
the effective signal--to--noise ratio we produced two Doppler maps. These show
similar features to maps of other rapidly rotating T Tauri stars, i.e. a polar
spot with more spots extending out of it down to equator. Comparison of the two
maps was carried out to measure the differential rotation. Cross-correlation
and parameter fitting indicates that TWA 6 does not have detectable
differential rotation.
The Balmer emission of the star was studied. The mean H-alpha profile has a
narrow component consistent with rotational broadening and a broad component
extending out to 250km/s. The variability in H-alpha suggests that the
chromosphere has active regions that are cospatial with the spots in the
photosphere, similar to the 'plages' observed on the Sun. In addition the star
has at least one slingshot prominence 3 stellar radii above the surface - the
first such detection in a T Tauri star.Comment: 13 pages, 16 figure
A Multi-Wavelength Perspective of Flares on HR 1099: Four Years of Coordinated Campaigns
We report on four years of multiple wavelength observations of the RS CVn
system V711 Tau (HR 1099) from 1993, 1994, 1996, and 1998. This combination of
radio, ultraviolet, extreme ultraviolet, and X-ray observations allows us to
view, in the most comprehensive manner currently possible, the coronal and
upper atmospheric variability of this active binary system. We report on the
changing activity state of the system as recorded in the EUV and radio across
the four years of the observations, and study the high energy variability using
an assemblage of X-ray telescopes. (Longer abstract in paper).Comment: manuscript is 110 pages in length; 36 figures tota
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