175 research outputs found
Nonlinear Outcome of Gravitational Instability in Disks with Realistic Cooling
We consider the nonlinear outcome of gravitational instability in optically
thick disks with a realistic cooling function. We use a numerical model that is
local, razor-thin, and unmagnetized. External illumination is ignored. Cooling
is calculated from a one-zone model using analytic fits to low temperature
Rosseland mean opacities. The model has two parameters: the initial surface
density Sigma_0 and the rotation frequency Omega. We survey the parameter space
and find: (1) The disk fragments when t_c,eff Omega = 1, where t_c,eff is an
effective cooling time defined as the average internal energy of the model
divided by the average cooling rate. This is consistent with earlier results
that used a simplified cooling function. (2) The initial cooling time t_c0 or a
uniform disk with Q = 1 can differ by orders of magnitude from t_c,eff in the
nonlinear outcome. The difference is caused by sharp variations in the opacity
with temperature. The condition t_c0 Omega = 1 therefore does not necessarily
indicate where fragmentation will occur. (3) The largest difference between
t_c,eff and t_c0 is near the opacity gap, where dust is absent and hydrogen is
largely molecular. (4) In the limit of strong illumination the disk is
isothermal; we find that an isothermal version of our model fragments for Q <
1.4. Finally, we discuss some physical processes not included in our model, and
find that most are likely to make disks more susceptible to fragmentation. We
conclude that disks with t_c,eff Omega < 1 do not exist.Comment: 30 pages, 12 figure
Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high resolution optical spectra obtained with the HIRES
spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs
(LMTTs) in Taurus-Auriga. Of particular interest is the previously classified
"continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass
just below the stellar/substellar boundary. None of the LMTTs in Taurus are
rapidly rotating (vsini < 30 km/s), unlike low mass objects in Orion. Many of
the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent
H-alpha emission (EWs of 3 - 36 A), indicative of active chromospheres. We
demonstrate empirically that the full-width at 10% of the H-alpha emission
profile peak is a more practical and possibly more accurate indicator of
accretion than either the equivalent width of H-alpha or optical veiling:
10%-widths > 270 km/s are classical T Tauri stars (i.e. accreting), independent
of stellar spectral type. Although LMTTs can have accretion rates comparable to
that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass
accretion rate appears to decrease with decreasing mass. The diminished
frequency of accretion disks for LMTTs, in conjunction with their lower, on
average, mass accretion rates, implies that they are formed with less massive
disks than higher-mass T Tauri stars. The radial velocities, circumstellar
properties and known binaries do not support the suggestion that many of the
lowest mass members of Taurus have been ejected from higher stellar density
regions within the cloud. Instead, LMTTs appear to have formed and are evolving
in the same way as higher-mass T Tauri stars, but with smaller disks and
shorter disk lifetimes.Comment: 27 pages, plus 8 figures, accepted for publication in Ap
VLT observations of GRB 990510 and its environment
We present BVRI photometry and spectrophotometry of GRB990510 obtained with
the ESO VLT/Antu telescope during the late decline phase. Between days 8 and 29
after the burst, the afterglow faded from R=24.2 to ~26.4. The spectral flux
distribution and the light curve support the interpretation of the afterglow as
synchrotron emission from a jet. The light curve is consistent with the optical
transient alone but an underlying SN with maximum brightness R>27.4 or a galaxy
with R>27.6 (3-sigma upper limits) cannot be ruled out. To a 5-sigma detection
threshold of R=26.1, no galaxy is found within 6'' of the transient. A very
blue V~24.5 extended object which may qualify as a starburst galaxy is located
12'' SE, but at unknown redshift.Comment: 5 pages A&A Latex, accepted for publication in A&A Letter
The white dwarf revealed in the intermediate polar V709 Cassiopeiae
Results are presented from the first detailed spectroscopic observations of
the recently identified intermediate polar RXJ0028.8+5917/V709 Cas. The study
of the emission line radial velocities allows us to remove the uncertainties on
the different aliases of the orbital period and a best value is found at
(0.2225 +/- 0.0002) day. It is also found that the system shows significant EW~
(2-4)A, broad absorptions affecting the Balmer lines from Hdelta to Hbeta.
These broad absorptions are interpreted as the contribution of an underlying
white dwarf atmosphere. The characteristics of the absorptions are found to be
consistent with a DA log(g) = 8 white dwarf at a temperature of ~ 23 000 K,
contributing ~ 17 % (at 4500 A) to the overall flux. This is the first direct
detection of a white dwarf in an intermediate polar system. The absence of
significant Zeeman splitting indicates a magnetic field lower than 10 MG,
confirming that, at least in some cases, intermediate polars have weaker fields
than polars. Different possibilities are discussed to explain the substantial
contribution of the white dwarf to the overall flux.Comment: 6 pages, 4 figures, 3 tables. accepted in Astronomy & Astrophysic
High angular resolution imaging and infrared spectroscopy of CoRoT candidates
Studies of transiting extrasolar planets are of key importance for
understanding the nature of planets outside our solar system because their
masses, diameters, and bulk densities can be measured. An important part of
transit-search programmes is the removal of false-positives. The critical
question is how many of the candidates that passed all previous tests are false
positives. For our study we selected 25 CoRoT candidates that have already been
screened against false-positives using detailed analysis of the light curves
and seeing-limited imaging, which has transits that are between 0.7 and 0.05%
deep. We observed 20 candidates with the adaptive optics imager NaCo and 18
with the high-resolution infrared spectrograph CRIRES. We found previously
unknown stars within 2 arcsec of the targets in seven of the candidates. All of
these are too faint and too close to the targets to have been previously
detected with seeing-limited telescopes in the optical. Our study thus leads to
the surprising results that if we remove all candidates excluded by the
sophisticated analysis of the light-curve, as well as carrying out deep imaging
with seeing-limited telescopes, still 28-35% of the remaining candidates are
found to possess companions that are bright enough to be false-positives. Given
that the companion-candidates cluster around the targets and that the J-K
colours are consistent with physical companions, we conclude that the
companion-candidates are more likely to be physical companions rather than
unrelated field stars.Comment: 12 pages, 12 figures, A&A in pres
Flickering in FU Orionis
We analyze new and published optical photometric data of FU Orionis, an
eruptive pre-main sequence star. The outburst consists of a 5.5 mag rise at B
with an e-folding timescale of roughly 50 days. The rates of decline at B and V
are identical, 0.015 +- 0.001 mag per yr. Random fluctuations superimposed on
this decline have an amplitude of 0.035 +- 0.005 mag at V and occur on
timescales of 1 day or less. Correlations between V and the color indices U-B,
B-V, and V-R indicate that the variable source has the optical colors of a G0
supergiant. We associate this behavior with small amplitude flickering of the
inner accretion disk.Comment: 19 pages of text, 3 tables, and 6 figures to be published in the
Astrophysical Journal, 10 March 200
Accretion-Induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aur
It is widely accepted that much of the stochastic variability of T Tauri
stars is due to accretion by a circumstellar disk. The emission line spectrum
as well as the excess continuum emission are common probes of this process. In
this communication, we present additional probes of the circumstellar
environment in the form of resonance lines of low ionization potential
elements. Using a set of 14 high resolution echelle observations of the
classical T Tauri star (CTTS), RW Aur, taken between 1986 and 1996, we
carefully measure the continuum veiling at each epoch by comparing more than
500 absorption lines with those of an appropriate template. This allows us to
accurately subtract out the continuum emission and to recover the underlying
photospheric spectrum. In doing so, we find that selected photospheric lines
are enhanced by the accretion process, namely the resonance lines of LiI and
KI. A resonance line of TiI and a low excitation potential line of CaI also
show weak enhancements. Simple slab models and computed line bisectors lead us
to propose that these line enhancements are markers of cool gas at the
beginning of the accretion flow which provides an additional source of line
opacity. These results suggest that published values of surface lithium
abundances of classical T Tauri stars are likely to be overestimated. This
would account for the various reports of surface lithium abundances in excess
of meteoritic values among the extreme CTTS. Computing LTE lithium abundances
of RW Aur in a low and then high accretion state yields abundances which vary
by one order of magnitude. The low accretion state lithium abundance is
consistent with theoretical predictions for a star of this age and mass while
the high accretion state spectrum yields a super-meteoritic lithium abundance.Comment: 28 pages, 8 figures, accepted by Ap
Inferring statistics of planet populations by means of automated microlensing searches
(abridged) The study of other worlds is key to understanding our own, and not
only provides clues to the origin of our civilization, but also looks into its
future. Rather than in identifying nearby systems and learning about their
individual properties, the main value of the technique of gravitational
microlensing is in obtaining the statistics of planetary populations within the
Milky Way and beyond. Only the complementarity of different techniques
currently employed promises to yield a complete picture of planet formation
that has sufficient predictive power to let us understand how habitable worlds
like ours evolve, and how abundant such systems are in the Universe. A
cooperative three-step strategy of survey, follow-up, and anomaly monitoring of
microlensing targets, realized by means of an automated expert system and a
network of ground-based telescopes is ready right now to be used to obtain a
first census of cool planets with masses reaching even below that of Earth
orbiting K and M dwarfs in two distinct stellar populations, namely the
Galactic bulge and disk. The hunt for extra-solar planets acts as a principal
science driver for time-domain astronomy with robotic-telescope networks
adopting fully-automated strategies. Several initiatives, both into facilities
as well as into advanced software and strategies, are supposed to see the
capabilities of gravitational microlensing programmes step-wise increasing over
the next 10 years. New opportunities will show up with high-precision
astrometry becoming available and studying the abundance of planets around
stars in neighbouring galaxies becoming possible. Finally, we should not miss
out on sharing the vision with the general public, and make its realization to
profit not only the scientists but all the wider society.Comment: 10 pages in PDF format. White paper submitted to ESA's Exo-Planet
Roadmap Advisory Team (EPR-AT); typos corrected. The embedded figures are
available from the author on request. See also "Towards A Census of
Earth-mass Exo-planets with Gravitational Microlensing" by J.P. Beaulieu, E.
Kerins, S. Mao et al. (arXiv:0808.0005
Discovery of a cataclysmic variable with a sub-stellar companion
We find that the ROSAT source 1RXS J105010.3-140431 is a cataclysmic variable
with orbital period of 88.6 minutes and a spectrum closely resembling WZ Sge.
In particular, emission lines are flanked by Stark-broadened absorption wings
probably originating in the photosphere of a compact object. The Balmer
absorption lines can be modeled by the spectrum of a DA white dwarf with 13 000
24 000 K. The strong absorption lines allowed us to obtain direct
radial velocities of the white dwarf using the cross-correlation technique. We
find an extremely low white dwarf radial velocity half amplitude, = 4
1 km s. This is consistent with the upper limit obtained from the
H emission line wing K < 20 km s. The corresponding mass
function is incompatible with a main sequence secondary, but is compatible with
a post orbital period minimum cataclysmic variable with a brown dwarf-like
secondary. The formal solution gives a secondary mass of 10-20 jovian masses.
Doppler maps for the emission lines and the hypothesis of black-body emission
indicate a steady state (T ) accretion disk mainly emitting in
H and an optically thicker hotspot with a strong contribution to the
higher order Balmer lines and \ion{He}{I} 5875. As in other long cycle length
dwarf novae, evidence for inner disk removal is found from the analysis of the
emission lines.Comment: 14 figures, 2 of them composed. Total 20 figures. Accepted for
publication in Astronomy and Astrophysic
The system parameters of DW Ursae Majoris
We present new constraints on the system parameters of the SW Sextantis star
DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the
Hubble Space Telescope. Our data were obtained during a low state of the
system, in which the UV light was dominated by the hot white dwarf (WD)
primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set
firm limits on the masses and radii of the system components and the distance
between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014,
M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5
the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's
mass-period relation for CV secondaries, our estimates for the system
parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001,
M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82
\pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral
type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160
pc, from time-resolved I- and K-band photometry. Finally, we have repeated
Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa
in order to account for the higher surface gravity indicated by our eclipse
analysis. In this way we obtained a second estimate for the distance, d = 590
\pm 100 pc, which allows us to obtain a second estimate for the spectral type
of the secondary, M7 \pm 2.0. We conclude that the true value for the distance
and spectral type will probably be in between the values obtained by the two
methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication
in Ap
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