150 research outputs found
The line-of-sight distribution of the gas in the inner 60 pc of the Galaxy
2MASS K_S band data of the inner 60 pc of the Galaxy are used to reconstruct
the line-of-sight distances of the giant molecular clouds located in this
region. Using the 2MASS H band image of the same region, two different
populations of point sources are identified according to their flux ratio in
the two bands. The population of blue point sources forms a homogeneous
foreground that has to be subtracted before analyzing the K_S band image. The
reconstruction is made using two basic assumptions: (i) an axis-symmetric
stellar distribution in the region of interest and
(ii) optically thick clouds with an area filling factor of ~1 that block all
light of stars located behind them. Due to the reconstruction method, the
relative distance between the different cloud complexes is a robust result,
whereas it is not excluded that the absolute distance with respect to Sgr A* of
structures located more than 10 pc in front of Sgr A* are understimated by up
to a factor of 2. It is shown that all structures observed in the 1.2 mm
continuum and in the CS(2-1) line are present in absorption. We place the 50 km
s^-1 cloud complex close to, but in front of, Sgr A*. The 20 km s^-1 cloud
complex is located in front of the 50 km s^-1 cloud complex and has a large LOS
distance gradient along the direction of the galactic longitude. The bulk of
the Circumnuclear Disk is not seen in absorption. This leads to an upper limit
of the cloud sizes within the Circumnuclear Disk of ~0.06 pc.Comment: 12 pages with 17 figures. Accepted for publication in A&
Large Area Mapping at 850 Microns. IV. Analysis of the Clump Distribution in the Orion B South Molecular Cloud
We present results from a survey of a 1300 arcmin^2 region of the Orion B
South molecular cloud, including NGC 2024, NGC 2023, and the Horsehead Nebula
(B33), obtained using the Submillimetre Common-User Bolometer Array (SCUBA) on
the James Clerk Maxwell Telescope. Submillimeter continuum observations at 450
microns and 850 microns are discussed. Using an automated algorithm, 57
discrete emission features (``clumps'') are identified in the 850 micron map.
The physical conditions within these clumps are investigated under the
assumption that the objects are in quasi-hydrostatic equilibrium. The best fit
dust temperature for the clumps is found to be T_d = 18 +/- 4 K, with the
exception of those associated with the few known far infrared sources residing
in NGC 2024. The latter internally heated sources are found to be much warmer.
In the region surrounding NGC 2023, the clump dust temperatures agree with
clump gas temperatures determined from molecular line excitation measurements
of the CO molecule. The bounding pressure on the clumps lies in the range
log(k^-1 P cm^3 K^-1) = 6.1 +/- 0.3. The cumulative mass distribution is steep
at the high mass end, as is the stellar Initial Mass Function. The distribution
flattens significantly at lower masses, with a turn-over around 3 -- 10 M_sun.Comment: 41 pages, 16 figures, accepted by Ap
The Galactic Magnetic Field's Effect in Star-Forming Region
We investigate the effect of the Milky Way's magnetic field in star forming
regions using archived 350 micron polarization data on 52 Galactic star
formation regions from the Hertz polarimeter module. The polarization angles
and percentages for individual telescope beams were combined in order to
produce a large-scale average for each source and for complexes of sources. In
more than 80% of the sources, we find a meaningful mean magnetic field
direction, implying the existence of an ordered magnetic field component at the
scale of these sources. The average polarization angles were analyzed with
respect to the Galactic coordinates in order to test for correlations between
polarization percentage, polarization angle, intensity, and Galactic location.
No correlation was found, which suggests that the magnetic field in dense
molecular clouds is decoupled from the large-scale Galactic magnetic field.
Finally, we show that the magnetic field directions in the complexes are
consistent with a random distribution on the sky
Abundances of Molecular Species in Barnard 68
Abundances for 5 molecules (C18O, CS, NH3, H2CO, and C3H2) and 1 molecular
ion (N2H+) and upper limits for the abundances of 1 molecule (13CO) and 1
molecular ion (HCO+) are derived for gas within the Bok globule Barnard 68
(B68). The abundances were determined using our own BIMA millimeter
interferometer data and single-dish data gathered from the literature, in
conjunction with a Monte Carlo radiative transfer model. Since B68 is the only
starless core to have its density structure strongly constrained via extinction
mapping, a major uncertainty has been removed from these determinations. All
abundances for B68 are lower than those derived for translucent and cold dense
clouds, but perhaps only significantly for N2H+, NH3, and C3H2. Depletion of CS
toward the extinction peak of B68 is hinted at by the large offset between the
extinction peak and the position of maximum CS line brightness. Abundances
derived here for C18O and N2H+ are consistent with other, recently determined
values at positions observed in common.Comment: 16 pages, 1 figure, accepted by AJ, typo corrected, reference removed
in Section 4.
Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
Context: The CARMENES survey is a high-precision radial velocity (RV)
programme that aims to detect Earth-like planets orbiting low-mass stars.
Aims: We develop least-squares fitting algorithms to derive the RVs and
additional spectral diagnostics implemented in the SpEctrum Radial Velocity
Analyser (SERVAL), a publicly available python code.
Methods: We measured the RVs using high signal-to-noise templates created by
coadding all available spectra of each star.We define the chromatic index as
the RV gradient as a function of wavelength with the RVs measured in the
echelle orders. Additionally, we computed the differential line width by
correlating the fit residuals with the second derivative of the template to
track variations in the stellar line width.
Results: Using HARPS data, our SERVAL code achieves a RV precision at the
level of 1m/s. Applying the chromatic index to CARMENES data of the active star
YZ CMi, we identify apparent RV variations induced by stellar activity. The
differential line width is found to be an alternative indicator to the commonly
used full width half maximum.
Conclusions: We find that at the red optical wavelengths (700--900 nm)
obtained by the visual channel of CARMENES, the chromatic index is an excellent
tool to investigate stellar active regions and to identify and perhaps even
correct for activity-induced RV variations.Comment: 13 pages, 13 figures. A&A in press. Code is available at
https://github.com/mzechmeister/serva
Testing the locality of transport in self-gravitating accretion discs - II. The massive disc case
In this paper, we extend our previous analysis (Lodato & Rice 2004) of the
transport properties induced by gravitational instabilities in cooling, gaseous
accretion discs to the case where the disc mass is comparable to the central
object. In order to do so, we have performed global, three-dimensional smoothed
particle hydrodynamics simulations of massive discs. These new simulations show
a much more complex temporal evolution with respect to the less massive case.
Whereas in the low disc mass case a self-regulated, marginally stable state
(characterized by an approximately constant radial profile of the stability
parameter ) is easily established, in the high disc mass case we observe the
development of an initial transient and subsequent settling down in a
self-regulated state in some simulations, or a series or recurrent spiral
episodes, with low azimuthal wave number , in others. Accretion in this last
case can therefore be a highly variable process. On the other hand, we find
that the secular evolution of the disc is relatively slow. In fact, the
time-average of the stress induced by self-gravity results in accretion
time-scales much longer than the dynamical timescale, in contrast with previous
isothermal simulations of massive accretion discs. We have also compared the
resulting stress tensor with the expectations based on a local theory of
transport, finding no significant evidence for global wave energy transport.Comment: MNRAS, accepted (higher quality, colour figures can be found at
http://www.ast.cam.ac.uk/~giuseppe/Publications/Locality_II/
Complex Structure in Class 0 Protostellar Envelopes II: Kinematic Structure from Single-Dish and Interferometric Molecular Line Mapping
We present a study of dense molecular gas kinematics in seventeen nearby
protostellar systems using single-dish and interferometric molecular line
observations. The non-axisymmetric envelopes around a sample of Class 0/I
protostars were mapped in the N2H+ (J=1-0) tracer with the IRAM 30m, CARMA and
PdBI as well as NH3 (1,1) with the VLA. The molecular line emission is used to
construct line-center velocity and linewidth maps for all sources to examine
the kinematic structure in the envelopes on spatial scales from 0.1 pc to ~1000
AU. The direction of the large-scale velocity gradients from single-dish
mapping is within 45 degrees of normal to the outflow axis in more than half
the sample. Furthermore, the velocity gradients are often quite substantial,
the average being ~2.3 km\s\pc. The interferometric data often reveal
small-scale velocity structure, departing from the more gradual large-scale
velocity gradients. In some cases, this likely indicates accelerating infall
and/or rotational spin-up in the inner envelope; the median velocity gradient
from the interferometric data is ~10.7 km/s/pc. In two systems, we detect
high-velocity HCO+ (J=1-0) emission inside the highest-velocity \nthp\
emission. This enables us to study the infall and rotation close to the disk
and estimate the central object masses. The velocity fields observed on large
and small-scales are more complex than would be expected from rotation alone,
suggesting that complex envelope structure enables other dynamical processes
(i.e. infall) to affect the velocity field.Comment: 85 Pages, 31 Figures, 11 Tables, Accepted to ApJ
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
Determining the Parameters of Massive Protostellar Clouds via Radiative Transfer Modeling
A one-dimensional method for reconstructing the structure of prestellar and
protostellar clouds is presented. The method is based on radiative transfer
computations and a comparison of theoretical and observed intensity
distributions at both millimeter and infrared wavelengths. The radiative
transfer of dust emission is modeled for specified parameters of the density
distribution, central star, and external background, and the theoretical
distribution of the dust temperature inside the cloud is determined. The
intensity distributions at millimeter and IR wavelengths are computed and
quantitatively compared with observational data. The best-fit model parameters
are determined using a genetic minimization algorithm, which makes it possible
to reveal the ranges of parameter degeneracy as well. The method is illustrated
by modeling the structure of the two infrared dark clouds IRDC-320.27+029 (P2)
and IRDC-321.73+005 (P2). The derived density and temperature distributions can
be used to model the chemical structure and spectral maps in molecular lines.Comment: Accepted for publication in Astronomy Report
Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy
We review the present observational knowledge on the spatial distribution and
the physical state of the different (molecular, atomic and ionized) components
of the interstellar gas in the innermost 3 kpc of our Galaxy -- a region which
we refer to as the interstellar Galactic bulge, to distinguish it from its
stellar counterpart. We try to interpret the observations in the framework of
recent dynamical models of interstellar gas flows in the gravitational
potential of a barred galaxy. Finally, relying on both the relevant
observations and their theoretical interpretation, we propose a model for the
space-averaged density of each component of the interstellar gas in the
interstellar Galactic bulge.Comment: 19 pages, 11 figure
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