1,152 research outputs found
UVES spectra of young brown dwarfs in Cha I: radial and rotational velocities
Based on high-resolution UVES spectra we found that the radial velocity (RV)
dispersion of nine of twelve known young bona fide and candidate brown dwarfs
in the Cha I dark cloud is 2.0 km/s, i.e. significantly smaller than the RV
dispersion of T Tauri stars in Cha I (3.6 km/s) and only slightly larger than
the dispersion of the surrounding molecular gas (1.2 km/s) (Mizuno et al.
1999). This result indicates that the majority of these brown dwarfs are not
ejected with high velocity out of a dense region as proposed by some formation
scenarios for brown dwarfs. The mean RV values are consistent with the objects
being kinematic members of Cha I. The RV dispersion of the T Tauri stars
confined to the Cha I region is based on a compilation of T Tauri stars with
known RVs from the literature plus three T Tauri stars observed with UVES and
unpublished RVs for nine T Tauri stars. Time-resolved spectroscopy revealed RV
variations for five out of nine of the bona fide and candidate brown dwarfs in
Cha I, which could be due to orbiting planets or surface features. Furthermore
we derived rotational velocities vsin(i) and the Lithium 6708 \AA equivalent
width.Comment: A&A Letter, in pres
A very massive runaway star from Cygnus OB2
Aims: We analyze the available information on the star BD+43 3654 to
investigate the possibility that it may have had its origin in the massive OB
association Cygnus OB2.
Methods: We present new spectroscopic observations allowing a reliable
spectral classification of the star, and discuss existing MSX observations of
its associated bow shock and astrometric information not previously studied.
Results: Our observations reveal that BD+43 3654 is a very early and luminous
star of spectral type O4If, with an estimated mass of (70 +/- 15) solar masses
and an age of about 1.6 Myr. The high spatial resolution of the MSX
observations allows us to determine its direction of motion in the plane of the
sky by means of the symmetry axis of the well-defined bow shock, which matches
well the orientation expected from the proper motion. Tracing back its path
across the sky we find that BD+43 3654 was located near the central, densest
region of Cygnus OB2 at a time in the past similar to its estimated age.
Conclusions: BD+43 3654 turns out to be one of the three most massive runaway
stars known, and it most likely formed in the central region of Cygnus OB2. A
runaway formation mechanism by means of dynamical ejection is consistent with
our results.Comment: Accepted by Astronomy and Astrophysics (letters); 5 pages, 3 figure
About the potential of lidars with different photodetectors under daytime sky radiation
Results of theoretical analysis and experimental developments implemented as advanced methods and means to improve a noise-immunity of lidar systems for practical applications are discussed. A particular attention is paid to assessing the developed methods and technical solutions effectiveness and their comparison with existing lidar systems and real receivers. © 2016 Owned by the authors, published by EDP Sciences.Peer ReviewedPostprint (published version
A prediction on the age of thick discs as a function of the stellar mass of the host galaxy
One of the suggested thick disc formation mechanisms is that they were born
quickly and in situ from a turbulent clumpy disc. Subsequently, thin discs
formed slowly within them from leftovers of the turbulent phase and from
material accreted through cold flows and minor mergers. In this letter, I
propose an observational test to verify this hypothesis. By combining thick
disc and total stellar masses of edge-on galaxies with galaxy stellar mass
functions calculated in the redshift range of , I derived a positive
correlation between the age of the youngest stars in thick discs and the
stellar mass of the host galaxy; galaxies with a present-day stellar mass of
have thick disc stars as
young as , whereas the youngest stars in the thick discs of
Milky-Way-like galaxies are old. I tested this prediction
against the scarcely available thick disc age estimates, all of them are from
galaxies with , and I
find that field spiral galaxies seem to follow the expectation. On the other
hand, my derivation predicts ages that are too low for the thick discs in
lenticular galaxies, indicating a fast early evolution for S0 galaxies. I
propose the idea of conclusively testing whether thick discs formed quickly and
in situ by obtaining the ages of thick discs in field galaxies with masses of
and by checking whether
they contain -old stars.Comment: Letter accepted for publication in A&
A monolithic collapse origin for the thin/thick disc structure of ESO 243-49
ESO 243-49 is a high-mass (circular velocity ) edge-on S0 galaxy in the Abell 2877 cluster at a distance of
. To elucidate the origin of its thick disc, we use MUSE
science verification data to study its kinematics and stellar populations. The
thick disc emits of the light at heights in excess of
(). The rotation velocities of its stars
lag by compared to those in the thin disc, which is
compatible with the asymmetric drift. The thick disc is found to be more
metal-poor than the thin disc, but both discs have old ages. We suggest an
internal origin for the thick disc stars in high-mass galaxies. We propose that
the thick disc formed either first in a turbulent phase with a high
star formation rate and that a thin disc formed shortly afterwards, or because of the dynamical heating of a thin pre-existing component. Either
way, the star formation in ESO 243-49 was quenched just a few Gyrs after the
galaxy was born and the formation of a thin and a thick disc must have occurred
before the galaxy stopped forming stars. The formation of the discs was so fast
that it could be described as a monolithic collapse where several generations
of stars formed in a rapid succession.Comment: Accepted for publication in A&A. The reduced data-cube as well as the
data necessary to build the kinematic and stellar population maps are
available at
https://etsin.avointiede.fi/dataset/urn-nbn-fi-csc-kata2016092414291163237
The Massive Star Forming Region, Cygnus OB2
We present results from a catalogue of 1696 X-ray point sources detected in
the massive star forming region (SFR) Cygnus OB2, the majority of which have
optical or near-IR associations. We derive ages of 3.5 and 5.25 Myrs for the
stellar populations in our two fields, in agreement with recent studies that
suggest that the central 1-3 Myr OB association is surrounded and contaminated
by an older population with an age of 5-10 Myrs. The fraction of sources with
proto-planetary disks, as traced by K-band excesses, are unusually low. Though
this has previously been interpreted as due to the influence of the large
number of OB stars in Cyg OB2, contamination from an older population of stars
in the region could also be responsible. An initial mass function is derived
and found to have a slope of Gamma = -1.27, in agreement with the canonical
value. Finally we introduce the recently approved Chandra Cygnus OB2 Legacy
Survey that will image a 1 square degree area of the Cygnus OB2 association to
a depth of 120 ks, likely detecting ~10,000 stellar X-ray sources.Comment: 4 pages, 3 figures. To appear in the proceedings of IAU Symposium
266, Star Clusters: Basic Galactic Building Blocks Throughout Time and Space,
eds. R. de Grijs and J. Lepin
Study for the computational resolution of conservation equations of mass, momentum and energy
The main purpose of this project is the creation of a CFD program able to solve the Navier-Stokes equations. Before the realisation of this program, other programs solving different problems will be created, like a 2D heat conduction program, a potential flow program and a convection-diffusion program. After those are done and validated, the Navier-Stokes solving program will be done. This program must also be validated, so the results obtained are reliable
The Formation and Early Evolution of Low-mass Stars and Brown Dwarfs
The discovery of large numbers of young low-mass stars and brown dwarfs over
the last decade has made it possible to investigate star formation and early
evolution in a previously unexplored mass regime. In this review, we begin by
describing surveys for low-mass members of nearby associations, open clusters,
star-forming regions and the methods used to characterize their stellar
properties. We then use observations of these populations to test theories of
star formation and evolution at low masses. For comparison to the formation
models, we consider the initial mass function, stellar multiplicity,
circumstellar disks, protostellar characteristics, and kinematic and spatial
distributions at birth for low-mass stars and brown dwarfs. To test the
evolutionary models, we focus on measurements of dynamical masses and empirical
Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.Comment: Final published version at http://www.annualreviews.org/journal/astr
Exploring Brown Dwarf Disks
We discuss the spectral energy distribution of three very low mass objects in
Chamaeleon I for which ground-based spectroscopy and photometry as well as ISO
measurements in the mid-infrared are available (Comeron et al. 2000; Persi et
al. 2000). One of these stars (Cha Halpha1) is a bona-fide brown dwarf, with
mass 0.04-0.05 Msun. We show that the observed emission is very well described
by models of circumstellar disks identical to those associated to T Tauri
stars, scaled down to keep the ratio of the disk-to-star mass constant and to
the appropriate stellar parameters. This result provides a first indication
that the formation mechanism of T Tauri stars (via core contraction and
formation of an accretion disk) extends to objects in the brown dwarf mass
range.Comment: 4 pages, 4 figures, A&A Letters in pres
- …