172 research outputs found
The Initial Mass Function of the Stellar Association NGC 602 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations
We present our photometric study of the stellar association NGC 602 in the
wing of the Small Magellanic Cloud (SMC). The data were taken in the filters
F555W and F814W using the Advanced Camera for Surveys (ACS) on-board the Hubble
Space Telescope (HST). Photometry was performed using the ACS module of the
stellar photometry package DOLPHOT. We detected more than 5,500 stars with a
magnitude range of 14 \lsim m_{555} \lsim 28 mag. Three prominent stellar
concentrations are identified with star counts in the observed field, the
association NGC 602 itself, and two clusters, one of them not being currently
in any known catalog. The Color-Magnitude Diagrams (CMDs) of both clusters show
features typical for young open clusters, while that of the association reveals
bright main sequence (MS) and faint pre-main sequence (PMS) stars as the
members of the system. We construct the initial mass spectrum (IMS) of the
association by applying an age-independent method of counting the PMS stars
within evolutionary tracks, while for the bright MS stars we transform their
magnitudes to masses with the use of mass-luminosity relations. The IMS of NGC
602 is found to be well represented by a single-power law, corresponding to an
Initial Mass Function (IMF) of slope \Gamma\approx -1.2 for 1 \lsim M/M{\solar}
\lsim 45. This indicates that the shape of the IMF of a star forming system in
the SMC for stars with masses higher than 1 M{\solar} seems to be quite similar
to the field IMF in the solar neighborhood.Comment: Accepted for publication in ApJ, 13 pages, 14 figures, emulateapj.cls
LaTeX style, full resolution version available on
http://www.astro.uni-bonn.de/~dgoulier/Science/NGC602/ms.pd
Pre-galactic metal enrichment - The chemical signatures of the first stars
The emergence of the first sources of light at redshifts of z ~ 10-30
signaled the transition from the simple initial state of the Universe to one of
increasing complexity. We review recent progress in our understanding of the
formation of the first stars and galaxies, starting with cosmological initial
conditions, primordial gas cooling, and subsequent collapse and fragmentation.
We emphasize the important open question of how the pristine gas was enriched
with heavy chemical elements in the wake of the first supernovae. We conclude
by discussing how the chemical abundance patterns conceivably allow us to probe
the properties of the first stars and subsequent stellar generations, and allow
us to test models of early metal enrichment.Comment: 52 pages, 20 figures, clarifications, references added, accepted for
publication in the Reviews of Modern Physic
On the Influence of Uncertainties in Chemical Reaction Rates on Results of the Astrochemical Modelling
With the chemical reaction rate database UMIST95 (Millar et al. 1997) we
analyze how uncertainties in rate constants of gas-phase chemical reactions
influence the modelling of molecular abundances in the interstellar medium.
Random variations are introduced into the rate constants to estimate the
scatter in theoretical abundances. Calculations are performed for dark and
translucent molecular clouds where gas phase chemistry is adequate. Similar
approach was used by Pineau des Forets & Roueff (2000) for the study of
chemical bistability. All the species are divided into 6 sensitivity groups
according to the value of the scatter in their model abundances computed with
varied rate constants. It is shown that the distribution of species within
these groups depends on the number of atoms in a molecule and on the adopted
physical conditions. The simple method is suggested which allows to single out
reactions that are most important for the evolution of a given species.Comment: 4 pages. To appear in the proceedings of the 4th Cologne-Bonn Zermatt
Symposiu
Star Formation in Space and Time: Taurus-Auriga
To understand the formation of stellar groups, one must first document
carefully the birth pattern within real clusters and associations. In this
study of Taurus-Auriga, we combine pre-main-sequence ages from our own
evolutionary tracks with stellar positions from observational surveys. Aided by
the extensive, millimeter data on the molecular clouds, we develop a picture of
the region's history. Star formation began, at a relatively low level and in a
spatially diffuse manner, at least 10 Myr in the past. Within the last few
million years, new stars have been produced at an accelerating rate, almost
exclusively within a confined group of striated cloud filaments. The gas both
inside and around the filaments appears to be in force balance. Thus, the
appearance of the filaments is due to global, quasi-static contraction of the
parent cloud material. Gravity drives this contraction and shock dissipation
mediates it, but the internal motion of the gas does not appear to be
turbulent. The accelerating nature of recent star formation means that the
condensation of cloud cores is a threshold phenomenon, requiring a minimum
background density. Other, nearby cloud regions, including Lupus and
Chamaeleon, contain some locales that have attained this density, and others
that have not. In the latter, we find extensive and sometimes massive molecular
gas that is still devoid of young stars.Comment: 19 pages, 7 figures, to be published in ApJ - December 20, 200
Optical Magnetometry
Some of the most sensitive methods of measuring magnetic fields utilize
interactions of resonant light with atomic vapor. Recent developments in this
vibrant field are improving magnetometers in many traditional areas such as
measurement of geomagnetic anomalies and magnetic fields in space, and are
opening the door to new ones, including, dynamical measurements of bio-magnetic
fields, detection of nuclear magnetic resonance (NMR), magnetic-resonance
imaging (MRI), inertial-rotation sensing, magnetic microscopy with cold atoms,
and tests of fundamental symmetries of Nature.Comment: 11 pages; 4 figures; submitted to Nature Physic
Hubble Space Telescope NICMOS Polarization Observations of Three Edge-on Massive YSOs
Massive young stellar objects (YSOs), like low-mass YSOs, appear to be
surrounded by optically thick envelopes and/or disks and have regions, often
bipolar, that are seen in polarized scattered light at near-infrared
wavelengths. We are using the 0.2'' spatial resolution of NICMOS on Hubble
Space Telescope to examine the structure of the disks and outflow regions of
massive YSOs in star-forming regions within a few kpc of the Sun. Here we
report on 2 micron polarimetry of NGC 6334 V and S255 IRS1. NGC 6334 V consists
of a double-lobed bright reflection nebula seen against a dark region, probably
an optically thick molecular cloud. Our polarization measurements show that the
illuminating star lies ~ 2'' south of the line connecting the two lobes; we do
not detect this star at 2 micron, but there are a small radio source and a
mid-infrared source at this location. S255 IRS1 consists of two YSOs (NIRS1 and
NIRS3) with overlapping scattered light lobes and luminosities corresponding to
early B stars. Included in IRS1 is a cluster of stars from whose polarization
we determine the local magnetic field direction. Neither YSO has its scattered
light lobes aligned with this magnetic field. The line connecting the scattered
light lobes of NIRS1 is twisted symmetrically around the star; the best
explanation is that the star is part of a close binary and the outflow axis of
NIRS1 is precessing as a result of non-coplanar disk and orbit. The star NIRS3
is also offset from the line connecting its two scattered light lobes. We
suggest that all three YSOs show evidence of episodic ejection of material as
they accrete from dense, optically thick envelopes.Comment: 39 pages, 7 figures, 4 tables To be published in The Astrophysical
Journa
Resonant nonlinear magneto-optical effects in atoms
In this article, we review the history, current status, physical mechanisms,
experimental methods, and applications of nonlinear magneto-optical effects in
atomic vapors. We begin by describing the pioneering work of Macaluso and
Corbino over a century ago on linear magneto-optical effects (in which the
properties of the medium do not depend on the light power) in the vicinity of
atomic resonances, and contrast these effects with various nonlinear
magneto-optical phenomena that have been studied both theoretically and
experimentally since the late 1960s. In recent years, the field of nonlinear
magneto-optics has experienced a revival of interest that has led to a number
of developments, including the observation of ultra-narrow (1-Hz)
magneto-optical resonances, applications in sensitive magnetometry, nonlinear
magneto-optical tomography, and the possibility of a search for parity- and
time-reversal-invariance violation in atoms.Comment: 51 pages, 23 figures, to appear in Rev. Mod. Phys. in Oct. 2002,
Figure added, typos corrected, text edited for clarit
Tracing the Mass during Low-Mass Star Formation, IV: Observations and Modeling of the Submillimeter Continuum Emission from Class I Protostars
We present results from the observations and modeling of seventeen Class I
cores with the Submillimetre Common Users Bolometer Array (SCUBA) on the James
Clerk Maxwell Telescope (JCMT). By modeling the transfer of radiation through
the envelope for nine cores, we find, for a power law distribution
n(r)=n_f(r/r_f)^-p, the average and standard deviation p=1.6 +/- 0.4 and a
median of p=1.8. However, the inclusion of a disk or other point-like component
can cause the derived p to be shallower by as much as 0.5. In addition, we test
the Shu collapse model for our sources and discuss the application of simpler
analyses that derive a density power law distribution directly from the slope
of the intensity radial profile. The total mass of the envelope in our sample
has a range from 0.04 to 5.0 M_sun, but these masses disagree with the virial
masses derived from molecular line observations. Finally, we discuss the nature
of these sources in light of various evolutionary indicators and find that
T_bol and L_obs/L_smm are often inconsistent in distinguishing Class 0 from
Class I cores.Comment: Accepted to ApJS. 50 pages with 27 figures and 12 tables.
High-resolution figures at http://peggysue.as.utexas.edu/cyoung
Sequential Star Formation in RCW 34: A Spectroscopic Census of the Stellar Content of High-mass Star-forming Regions
We present VLT/SINFONI integral field spectroscopy of RCW 34 along with
Spitzer/IRAC photometry of the surroundings. RCW 34 consists of three different
regions. A large bubble has been detected on the IRAC images in which a cluster
of intermediate- and low-mass class II objects is found. At the northern edge
of this bubble, an HII region is located, ionized by 3 OB stars. Intermediate
mass stars (2 - 3 Msun) are detected of G- and K- spectral type. These stars
are still in the pre-main sequence (PMS) phase. North of the HII region, a
photon-dominated region is present, marking the edge of a dense molecular cloud
traced by H2 emission. Several class 0/I objects are associated with this
cloud, indicating that star formation is still taking place. The distance to
RCW 34 is revised to 2.5 +- 0.2 kpc and an age estimate of 2 - 1 Myrs is
derived from the properties of the PMS stars inside the HII region. The most
likely scenario for the formation of the three regions is that star formation
propagates from South to North. First the bubble is formed, produced by
intermediate- and low-mass stars only, after that, the HII region is formed
from a dense core at the edge of the molecular cloud, resulting in the
expansion as a champagne flow. More recently, star formation occurred in the
rest of the molecular cloud. Two different formation scenarios are possible:
(a) The bubble with the cluster of low- and intermediate mass stars triggered
the formation of the O star at the edge of the molecular cloud which in turn
induces the current star-formation in the molecular cloud. (b) An external
triggering is responsible for the star-formation propagating from South to
North. [abridged]Comment: 19 pages, 11 figures, accepted by Ap
The influence of ultra-high-energy cosmic rays on star formation in the early universe
The presence of ultra-high-energy cosmic rays (UHECR) results in an increase
in the degree of ionization in the post-recombination Universe, which
stimulates the efficiency of the production of H molecules and the
formation of the first stellar objects. As a result, the onset of the formation
of the first stars is shifted to higher redshifts, and the masses of the first
stellar systems decrease. As a consequence, a sufficient increase in the
ionizing radiation providing the reionization of the Universe can take place.
We discuss possible observational manifestations of these effects and their
dependence on the parameters of UHECR.Comment: 10 pages, 5 figure
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