4,011 research outputs found
X-rays and Protostars in the Trifid Nebula
The Trifid Nebula is a young HII region recently rediscovered as a
"pre-Orion" star forming region, containing protostars undergoing violent mass
ejections visible in optical jets as seen in images from the Infrared Space
Observatory and the Hubble Space Telescope. We report the first X-ray
observations of the Trifid nebula using ROSAT and ASCA. The ROSAT image shows a
dozen X-ray sources, with the brightest X-ray source being the O7 star, HD
164492, which provides most of the ionization in the nebula. We also identify
85 T Tauri star and young, massive star candidates from near-infrared colors
using the JHKs color-color diagram from the Two Micron All Sky Survey (2MASS).
Ten X-ray sources have counterpart near-infrared sources. The 2MASS stars and
X-ray sources suggest there are potentially numerous protostars in the young
HII region of the Trifid. ASCA moderate resolution spectroscopy of the
brightest source shows hard emission up to 10 keV with a clearly detected Fe K
line. The best model fit is a two-temperature (T = 1.2x10^6 K and 39x10^6 K)
thermal model with additional warm absorbing media. The hotter component has an
unusually high temperature for either an O star or an HII region; a typical
Galactic HII region could not be the primary source for such hot temperature
plasma and the Fe XXV line emission. We suggest that the hotter component
originates in either the interaction of the wind with another object (a
companion star or a dense region of the nebula) or from flares from deeply
embedded young stars.Comment: Accepted in ApJ (Oct, 20 issue, 2001
Eclipsing Binaries in the OGLE Variable Star Catalog. IV. The Pre-Contact, Equal-Mass Systems
We used the database of eclipsing binaries detected by the OGLE microlensing
project in the pencil-beam search volume toward Baade's Window to define a
sample of 74 detached, equal-mass, main-sequence binary stars with short
orbital periods in the range 0.19<P<8 days. The logarithmic slope of the period
distribution, logN propto (-0.8 pm 0.2) logP, was used to infer the
angular-momentum-loss (AML) efficiency for the late, rapidly-rotating members
of close binaries. It is very likely that the main cause of the negative slope
is a discovery selection bias that progressively increases with the orbital
period length. Assuming a power-law dependence for the correction for the bias:
bias propto -C logP (with C ge 0), the AML braking-efficiency exponent alpha in
dH/dt = P^-alpha can take any value alpha = (-1.1 pm 0.2) + C. Very simple
considerations of discovery biases suggest C simeq 4/3, which would give an AML
braking law very close to the "saturated" one, with no dependence on the
period. However, except for plausibility arguments, we have no firm data to
support this estimate of C, so that alpha remains poorly constrained. The
results signal the utmost importance of the detection bias evaluation for
variable star databases used in analyses similar to the one presented in this
study.Comment: accepted by AJ, October 1999. AASTEX-4. 9 PS figures and 3 table
An assessment of Li abundances in weak-lined and classical T Tauri stars of the Taurus-Auriga association
Accurate measurements of lithium abundances in young low-mass stars provide
an independent and reliable age diagnostics. Previous studies of nearby star
forming regions have identified significant numbers of Li-depleted stars,often
at levels inconsistent with the ages indicated by their luminosity. We aim at a
new and accurate analysis of Li abundances in a sample of ~100 pre-main
sequence stars in Taurus-Auriga using a homogeneous and updated set of stellar
parameters and model atmospheres appropriate for the spectral types of the
sample stars.We compute Li abundances using published values of the equivalent
widths of the Li 6708 A doublet obtained from medium/high resolution spectra.
We find that the number of significantly Li-depleted stars in Taurus-Auriga is
greatly reduced with respect to earlier results. Only 13 stars have abundances
lower than the interstellar value by a factor of 5 or greater. All of them are
weak-lined T Tauri stars drawn from X-ray surveys; with the exception of four
stars located near the L1551 and L1489 dark clouds, all the Li-depleted stars
belong to the class of dispersed low-mass stars, distributed around the main
sites of current star formation. If located at the distance of Taurus-Auriga,
the stellar ages implied by the derived Li abundances are in the range 3-30
Myr, greater than the bulk of the Li-rich population with implication on the
star formation history of the region. In order to derive firm conclusions about
the fraction of Li-depleted stars of Taurus-Auriga, Li measurements of the
remaining members of the association should be obtained, in particular of the
group of stars that fall in the Li-burning region of the HR diagram.Comment: Accepted for publication in Astronomy & Astrophysics. 20 pages, 5
figure
A proper motion study of the Lupus clouds using VO tools
Aims: By using kinematical information, we test the membership of the new
Lupus candidate members proposed by the Cores to Disks (c2d) Spitzer Legacy
Program program and by a complementary optical survey. We also investigate the
relationship between the proper motions (pm) and other properties, in order to
get some clues about their formation and early evolution. Methods: We compiled
a list of members and possible members of Lupus 1, 3, and 4, together with all
information on their spectral types, disks, and physical parameters. Using
VO-tools, we cross-matched this list with the astrometric catalogues to get pm.
Our final sample contains sources with magnitudes I<16mag and estimated masses
>~0.1Msun. Results: According to the kinematic information, our sources can be
divided into two main groups. The first one contains sources with higher pm in
agreement with other Gould Belt populations and with spatial distribution,
optical and near-infrared colours, and disk composition consistent with these
objects belonging to the Lupus clouds. In the second group, sources have lower
pm with random orientations, and they are mostly located outside the cloud
cores, making their association with the Lupus complex more doubtful. We
investigate the properties of the higher pm group, but cannot find any
correlations with spatial location, binarity, the presence of a circumstellar
disk, or with physical properties such as effective temperature, luminosity,
mass, or age. Conclusions: We conclude that the lower pm group probably
represents a background population or mixture of populations unrelated to the
Lupus clouds. The higher pm group, on the other hand, has properties consistent
with it being a genuine population of the Lupus star-forming region. More
accurate pm and/or radial velocity information are required for a more detailed
study of the kinematic properties of the Lupus stellar members.Comment: Accepted in A&
The Chromo-Dielectric Soliton Model: Quark Self Energy and Hadron Bags
The chromo-dielectric soliton model (CDM) is Lorentz- and chirally-invariant.
It has been demonstrated to exhibit dynamical chiral symmetry breaking and
spatial confinement in the locally uniform approximation. We here study the
full nonlocal quark self energy in a color-dielectric medium modeled by a two
parameter Fermi function. Here color confinement is manifest. The self energy
thus obtained is used to calculate quark wave functions in the medium which, in
turn, are used to calculate the nucleon and pion masses in the one gluon
exchange approximation. The nucleon mass is fixed to its empirical value using
scaling arguments; the pion mass (for massless current quarks) turns out to be
small but non-zero, depending on the model parameters.Comment: 24 pages, figures available from the author
muCool: A novel low-energy muon beam for future precision experiments
Experiments with muons () and muonium atoms () offer
several promising possibilities for testing fundamental symmetries. Examples of
such experiments include search for muon electric dipole moment, measurement of
muon and experiments with muonium from laser spectroscopy to gravity
experiments. These experiments require high quality muon beams with small
transverse size and high intensity at low energy.
At the Paul Scherrer Institute, Switzerland, we are developing a novel device
that reduces the phase space of a standard beam by a factor of
with efficiency. The phase space compression is achieved by
stopping a standard beam in a cryogenic helium gas. The stopped
are manipulated into a small spot with complex electric and magnetic
fields in combination with gas density gradients. From here, the muons are
extracted into the vacuum and into a field-free region. Various aspects of this
compression scheme have been demonstrated. In this article the current status
will be reported.Comment: 8 pages, 5 figures, TCP 2018 conference proceeding
The Angular Momentum Content and Evolution of Class I and Flat-Spectrum Protostars
We report on the angular momentum content of heavily embedded protostars
based on our analysis of the projected rotation velocities (v sin i s) of 38
Class I/flat spectrum young stellar objects presented by Doppmann et al (2005).
After correcting for projection effects, we find that infrared-selected Class
I/flat spectrum objects rotate significantly more quickly (median equatorial
rotation velocity ~ 38 km/sec) than Classical T Tauri stars (CTTSs; median
equatorial rotation velocity ~ 18 km/sec) in the Rho Ophiuchi and
Taurus-Aurigae regions. The detected difference in rotation speeds between
Class I/flat spectrum sources and CTTSs proves difficult to explain without
some mechanism which transfers angular momentum out of the protostar between
the two phases. Assuming Class I/flat spectrum sources possess physical
characteristics (M_*,R_*,B_*) typical of pre-main sequence stars, fully disk
locked Class I objects should have co-rotation radii within their protostellar
disks that match well (within 30%) with the predicted magnetic coupling radii
of Shu et al (1994). The factor of two difference in rotation rates between
Class I/flat spectrum and CTTS sources, when interpreted in the context of disk
locking models, also imply a factor of 5 or greater difference in mass
accretion rates between the two phases.Comment: 13 pages, 6 figures. Accepted for publication in the Astronomical
Journal (tentatively for June 2005 edition
The disk-bearing young star IM Lup
Classical T Tauri stars (CTTS) differ in their X-ray signatures from older
pre-main sequence stars, e.g. weak-lined TTS (WTTS). CTTS show a soft excess
and deviations from the low-density coronal limit in the He-like triplets. We
test whether these features correlate with accretion or the presence of a disk
by observing IM Lup, a disk-bearing object apparently in transition between
CTTS and WTTS without obvious accretion. We analyse a Chandra grating spectrum
and additional XMM-Newton data of IM Lup and accompanying optical spectra, some
of them taken simultaneously to the X-ray observations. We fit the X-ray
emission lines and decompose the Ha emission line in different components. In
X-rays IM Lup has a bright and hot active corona, where elements of low
first-ionisation potential are depleted. The He-like Ne IX triplet is in the
low-density state, but due to the small number of counts a high-density
scenario cannot be excluded on the 90% confidence level. In all X-ray
properties IM Lup resembles a main-sequence star, but it is also compatible
with CTTS signatures on the 90% confidence level, thus we cannot decide if the
soft excess and deviations from the low-density coronal limit in the He-like
triplets in CTTS require accretion or only the presence of a disk. IM Lup is
chromospherically active, which explains most of the emission in Ha. Despite
its low equivalent width, the complexity of the Ha line profile is reminiscent
of CTTS. We present an estimate for the mass accretion rate of 10e-11 Msun/yr.Comment: 10 pages, 6 figures, accepted by A&
Third-order Coulomb corrections to the S-wave Green function, energy levels and wave functions at the origin
We obtain analytic expressions for the third-order corrections due to the
strong interaction Coulomb potential to the S-wave Green function, energy
levels and wave functions at the origin for arbitrary principal quantum number
n. Together with the known non-Coulomb correction this results in the complete
spectrum of S-states up to order alpha_s^5. The numerical impact of these
corrections on the Upsilon spectrum and the top quark pair production cross
section near threshold is estimated.Comment: 24 pages, LaTeX, v2: eq.(30) corrected (-13/8->-15/8
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