4,886 research outputs found
Testing the existence of optical linear polarization in young brown dwarfs
Linear polarization can be used as a probe of the existence of atmospheric
condensates in ultracool dwarfs. Models predict that the observed linear
polarization increases withthe degree of oblateness, which is inversely
proportional to the surface gravity. We aimed to test the existence of optical
linear polarization in a sample of bright young brown dwarfs, with spectral
types between M6 and L2, observable from the Calar Alto Observatory, and
cataloged previously as low gravity objects using spectroscopy. Linear
polarimetric images were collected in I and R-band using CAFOS at the 2.2 m
telescope in Calar Alto Observatory (Spain). The flux ratio method was employed
to determine the linear polarization degrees. With a confidence of 3,
our data indicate that all targets have a linear polarimetry degree in average
below 0.69% in the I-band, and below 1.0% in the R-band, at the time they were
observed. We detected significant (i.e. P/ 3) linear polarization
for the young M6 dwarf 2MASS J04221413+1530525 in the R-band, with a degree of
= 0.81 0.17 %.Comment: Accepted for publication in MNRA
Properties and occurrence rates of exoplanet candidates as a function of host star metallicity from the DR25 catalog
Correlations between the occurrence rate of exoplanets and their host star
properties provide important clues about the planet formation processes. We
studied the dependence of the observed properties of exoplanets (radius, mass,
and orbital period) as a function of their host star metallicity. We analyzed
the planetary radii and orbital periods of over 2800 candidates from
the latest data release DR25 (Q1-Q17) with revised planetary radii
based on ~DR2 as a function of host star metallicity (from the Q1-Q17
(DR25) stellar and planet catalog). With a much larger sample and improved
radius measurements, we are able to reconfirm previous results in the
literature. We show that the average metallicity of the host star increases as
the radius of the planet increases. We demonstrate this by first calculating
the average host star metallicity for different radius bins and then
supplementing these results by calculating the occurrence rate as a function of
planetary radius and host star metallicity. We find a similar trend between
host star metallicity and planet mass: the average host star metallicity
increases with increasing planet mass. This trend, however, reverses for masses
: host star metallicity drops with increasing planetary
mass. We further examined the correlation between the host star metallicity and
the orbital period of the planet. We find that for planets with orbital periods
less than 10 days, the average metallicity of the host star is higher than that
for planets with periods greater than 10 days.Comment: 14 pages, 13 Figures, Accepted for publication in The Astronomical
Journa
New constraints on the formation and settling of dust in the atmospheres of young M and L dwarfs
We obtained medium-resolution near-infrared spectra of seven young M9.5-L3
dwarfs classified in the optical. We aim to confirm the low surface gravity of
the objects in the NIR. We also test whether atmospheric models correctly
represent the formation and the settling of dust clouds in the atmosphere of
young late-M and L dwarfs. We used ISAAC at VLT to obtain the spectra of the
targets. We compared them to those of mature and young BD, and young late-type
companions to nearby stars with known ages, in order to identify and study
gravity-sensitive features. We computed spectral indices weakly sensitive to
the surface gravity to derive near-infrared spectral types. Finally, we found
the best fit between each spectrum and synthetic spectra from the BT-Settl 2010
and 2013 models. Using the best fit, we derived the atmospheric parameters of
the objects and identify which spectral characteristics the models do not
reproduce. We confirmed that our objects are young BD and we found NIR spectral
types in agreement with the ones determined at optical wavelengths. The
spectrum of the L2-gamma dwarf 2MASSJ2322-6151 reproduces well the spectrum of
the planetary mass companion 1RXS J1609-2105b. BT-Settl models fit the spectra
and the 1-5 m SED of the L0-L3 dwarfs for temperatures between 1600-2000
K. But the models fail to reproduce the shape of the H band, and the NIR slope
of some of our targets. This fact, and the best fit solutions found with
super-solar metallicity are indicative of a lack of dust, in particular at high
altitude, in the cloud models. The modeling of the vertical mixing and of the
grain growth will be revised in the next version of the BT-Settl models. These
revisions may suppress the remaining non-reproducibilities.Comment: Accepted in A&A, February 6, 201
NGC 2264 IRS1: The central engine and its cavity
We present a high-resolution study of NGC 2264 IRS1 in CS(2-1) and in the
3-mm continuum using the IRAM Plateau de Bure Interferometer. We complement
these radio data with images taken at 2.2, 4.6, and 11.9 micron. The combined
information allow a new interpretation of the closest environment of NGC 2264
IRS1. No disk around the B-type star IRS1 was found. IRS1 and its low-mass
companions are located in a low-density cavity which is surrounded by the
remaining dense cloud core which has a clumpy shell-like structure. Strong
evidence for induced on-going star formation was found in the surroundings of
IRS1. A deeply embedded very young stellar object 20 arcsec to the north of
IRS1 is powering a highly collimated bipolar outflow. The object 8 in the
closer environment of IRS1 is a binary surrounded by dusty circumbinary
material and powering two bipolar outflows.Comment: 17 pages, 6 figures, The paper is accepted and will appear in the
Astrophysical Journal, Vol 599, No 1 (issue December 10). A high-resolution
postscript version of this paper is available here (
http://www.astro.uni-jena.de/Users/martin/publi.html). Furthermore, you can
find a high resolution PDF file here (
http://www.tls-tautenburg.de/research/tls-research/pub2003.html
Mid-infrared interferometry of massive young stellar objects
The very inner structure of massive young stellar objects (YSOs) is difficult
to trace. With conventional observational methods we identify structures still
several hundreds of AU in size. However, the (proto-)stellar growth takes place
at the innermost regions (<100 AU) where the actual mass transfer onto the
forming high-mass star occurs. We present results from our programme toward
massive YSOs at the VLTI, utilising the two-element interferometer MIDI. To
date, we observed 10 well-known massive YSOs down to scales of 20 mas
(typically corresponding to 20 - 40 AU for our targets) in the 8-13 micron
region. We clearly resolve these objects which results in low visibilities and
sizes in the order of 30-50 mas. For two objects, we show results of our
modelling. We demonstrate that the MIDI data can reveal decisive structure
information for massive YSOs. They are often pivotal in order to resolve
ambiguities still immanent in model parameters derived from sole SED fitting.Comment: 6 pages, 5 figures, necessary style files iopams.sty, jpconf11.clo,
and jpconf.cls included; contribution for the conference "The Universe under
the Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to
be published in Journal of Physics: Conference Series by Institute of Physics
Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.
The Arecibo L-band Feed Array Zone of Avoidance Survey I: Precursor Observations through the Inner and Outer Galaxy
The Arecibo L-band Feed Array (ALFA) is being used to conduct a low-Galactic
latitude survey, to map the distribution of galaxies and large-scale structures
behind the Milky Way through detection of galaxies' neutral hydrogen (HI) 21-cm
emission. This Zone of Avoidance (ZOA) survey finds new HI galaxies which lie
hidden behind the Milky Way, and also provides redshifts for partially-obscured
galaxies known at other wavelengths. Before the commencement of the full
survey, two low-latitude precursor regions were observed, totalling 138 square
degrees, with 72 HI galaxies detected. Detections through the inner Galaxy
generally have no cataloged counterparts in any other waveband, due to the
heavy extinction and stellar confusion. Detections through the outer Galaxy are
more likely to have 2MASS counterparts. We present the results of these
precursor observations, including a catalog of the detected galaxies, with
their HI parameters. The survey sensitivity is well described by a flux- and
linewidth-dependent signal-to-noise ratio of 6.5. ALFA ZOA galaxies which also
have HI measurements in the literature show good agreement between our
measurements and previous work. The inner Galaxy precursor region was chosen to
overlap the HI Parkes Zone of Avoidance Survey so ALFA performance could be
quickly assessed. The outer Galaxy precursor region lies north of the Parkes
sky. Low-latitude large-scale structure in this region is revealed, including
an overdensity of galaxies near l = 183 deg and between 5000 - 6000 km/s in the
ZOA. The full ALFA ZOA survey will be conducted in two phases: a shallow survey
using the observing techniques of the precursor observations, and also a deep
phase with much longer integration time, with thousands of galaxies predicted
for the final catalog.Comment: 26 pages, 7 figures, 2 tables, Astronomical Journal accepte
Characterization of the Benchmark Binary NLTT 33370
We report the confirmation of the binary nature of the nearby, very low-mass
system NLTT 33370 with adaptive optics imaging and present resolved
near-infrared photometry and integrated light optical and near-infrared
spectroscopy to characterize the system. VLT-NaCo and LBTI-LMIRCam images show
significant orbital motion between 2013 February and 2013 April. Optical
spectra reveal weak, gravity sensitive alkali lines and strong lithium 6708
Angstrom absorption that indicate the system is younger than field age.
VLT-SINFONI near-IR spectra also show weak, gravity sensitive features and
spectral morphology that is consistent with other young, very low-mass dwarfs.
We combine the constraints from all age diagnostics to estimate a system age of
~30-200 Myr. The 1.2-4.7 micron spectral energy distribution of the components
point toward T_eff=3200 +/- 500 K and T_eff=3100 +/- 500 K for NLTT 33370 A and
B, respectively. The observed spectra, derived temperatures, and estimated age
combine to constrain the component spectral types to the range M6-M8.
Evolutionary models predict masses of 113 +/- 8 M_Jup and 106 +/- 7 M_Jup from
the estimated luminosities of the components. KPNO-Phoenix spectra allow us to
estimate the systemic radial velocity of the binary. The Galactic kinematics of
NLTT 33370AB are broadly consistent with other young stars in the Solar
neighborhood. However, definitive membership in a young, kinematic group cannot
be assigned at this time and further follow-up observations are necessary to
fully constrain the system's kinematics. The proximity, age, and late-spectral
type of this binary make it very novel and an ideal target for rapid, complete
orbit determination. The system is one of only a few model calibration
benchmarks at young ages and very low-masses.Comment: 25 pages, 3 tables, 13 figures, accepted for publication in The
Astrophysical Journa
Dynamics of Resonances in Strongly Interacting Systems
The effects of the propagation of particles which have a finite life-time and
an according broad distribution in their mass spectrum are discussed in the
context of a transport descriptions. In the first part some example cases of
mesonic modes in nuclear matter at finite densities and temperatures are
presented. These equilibrium calculations illustrate the dynamical range of
spectral distributions to be adequately covered by non-equilibrium description
of the dynamics of two nuclei colliding at high energies. The second part
addresses the problem of transport descriptions which properly account for the
damping width of the particles. A systematic and general gradient approximation
is presented in the form of diagrammatic rules which permit to derive a
self-consistent transport scheme from the Kadanoff--Baym equation. The scheme
is conserving and thermodynamically consistent provided the self-energies are
obtained within the Phi-derivable two-particle irreducible (2PI) method of
Baym. The merits, the limitations and partial cures of the limitations of this
transport scheme are discussed in detail.Comment: To appear in the proceedings of the International Conference
"Progress in Nonequilibrium Green's Functions III", Kiel, 22.-26. August 200
A Self-Consistent Model of the Circumstellar Debris Created by a Giant Hypervelocity Impact in the HD172555 System
Spectral modeling of the large infrared excess in the Spitzer IRS spectra of
HD 172555 suggests that there is more than 10^19 kg of sub-micron dust in the
system. Using physical arguments and constraints from observations, we rule out
the possibility of the infrared excess being created by a magma ocean planet or
a circumplanetary disk or torus. We show that the infrared excess is consistent
with a circumstellar debris disk or torus, located at approximately 6 AU, that
was created by a planetary scale hypervelocity impact. We find that radiation
pressure should remove submicron dust from the debris disk in less than one
year. However, the system's mid-infrared photometric flux, dominated by
submicron grains, has been stable within 4 percent over the last 27 years, from
IRAS (1983) to WISE (2010). Our new spectral modeling work and calculations of
the radiation pressure on fine dust in HD 172555 provide a self-consistent
explanation for this apparent contradiction. We also explore the unconfirmed
claim that 10^47 molecules of SiO vapor are needed to explain an emission
feature at 8 um in the Spitzer IRS spectrum of HD 172555. We find that unless
there are 10^48 atoms or 0.05 Earth masses of atomic Si and O vapor in the
system, SiO vapor should be destroyed by photo-dissociation in less than 0.2
years. We argue that a second plausible explanation for the 8 um feature can be
emission from solid SiO, which naturally occurs in submicron silicate "smokes"
created by quickly condensing vaporized silicate.Comment: Accepted to the Astrophysical Journa
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