427 research outputs found
A Census of the Chamaeleon I Star-Forming Region
Optical spectroscopy has been obtained for 179 objects that have been
previously identified as possible members of the cluster, that lack either
accurate spectral types or clear evidence of membership, and that are optically
visible (I<18). I have used these spectroscopic data and all other available
constraints to evaluate the spectral classifications and membership status of a
total sample of 288 candidate members of Chamaeleon I that have appeared in
published studies of the cluster. The latest census of Chamaeleon I now
contains 158 members, 8 of which are later than M6 and thus are likely to be
brown dwarfs. I find that many of the objects identified as members of
Chamaeleon I in recent surveys are actually field stars. Meanwhile, 7 of 9
candidates discovered by Carpenter and coworkers are confirmed as members, one
of which is the coolest known member of Chamaeleon I at a spectral type of M8
(~0.03 M_sun). I have estimated extinctions, luminosities, and effective
temperatures for the members and used these data to construct an H-R diagram
for the cluster. Chamaeleon I has a median age of ~2 Myr according to
evolutionary models, and hence is similar in age to IC 348 and is slightly
older than Taurus (~1 Myr). The measurement of an IMF for Chamaeleon I from
this census is not possible because of the disparate methods with which the
known members were originally selected, and must await an unbiased,
magnitude-limited survey of the cluster.Comment: 59 pages, 22 figure
Constraining the Environment of CH+ Formation with CH3+ Observations
The formation of CH+ in the interstellar medium has long been an outstanding
problem in chemical models. In order to probe the physical conditions of the
ISM in which CH+ forms, we propose the use of CH3+ observations. The pathway to
forming CH3+ begins with CH+, and a steady state analysis of CH3+ and the
reaction intermediary CH2+ results in a relationship between the CH+ and CH3+
abundances. This relationship depends on the molecular hydrogen fraction, f_H2,
and gas temperature, T, so observations of CH+ and CH3+ can be used to infer
the properties of the gas in which both species reside. We present observations
of both molecules along the diffuse cloud sight line toward Cyg OB2 No. 12.
Using our computed column densities and upper limits, we put constraints on the
f_H2 vs. T parameter space in which CH+ and CH3+ form. We find that average,
static, diffuse molecular cloud conditions (i.e. f_H2>0.2, T~60 K) are excluded
by our analysis. However, current theory suggests that non-equilibrium effects
drive the reaction C+ + H_2 --> CH+ + H, endothermic by 4640 K. If we consider
a higher effective temperature due to collisions between neutrals and
accelerated ions, the CH3+ partition function predicts that the overall
population will be spread out into several excited rotational levels. As a
result, observations of more CH3+ transitions with higher signal-to-noise
ratios are necessary to place any constraints on models where magnetic
acceleration of ions drives the formation of CH+.Comment: 7 pages, 3 figures, 2 tables, accepted for publication in Ap
Evidence for dust evolution within the Taurus Complex from Spitzer images
We present Spitzer images of the Taurus Complex (TC) and take advantage of
the sensitivity and spatial resolution of the observations to characterize the
diffuse IR emission across the cloud. This work highlights evidence of dust
evolution within the translucent sections of the archetype reference for
studies of quiescent molecular clouds. We combine Spitzer 160 um and IRAS 100
um observations to produce a dust temperature map and a far-IR dust opacity map
at 5' resolution. The average dust temperature is about 14.5K with a dispersion
of +/-1K across the cloud. The far-IR dust opacity is a factor 2 larger than
the average value for the diffuse ISM. This opacity increase and the
attenuation of the radiation field (RF) both contribute to account for the
lower emission temperature of the large grains. The structure of the TC
significantly changes in the mid-IR images that trace emission from PAHs and
VSGs. We focus our analysis of the mid-IR emission to a range of ecliptic
latitudes where the zodiacal light residuals are small. Within this cloud area,
there are no 8 and 24 um counterparts to the brightest 160 um emission
features. Conversely, the 8 and 24 um images reveal filamentary structure that
is strikingly inconspicuous in the 160 um and extinction maps. The IR colors
vary over sub-parsec distances across this filamentary structure. We compare
the observed colors with model calculations quantifying the impact of the RF
intensity and the abundance of stochastically heated particles on the dust SED.
To match the range of observed colors, we have to invoke variations by a factor
of a few of both the interstellar RF and the abundance of PAHs and VSGs. We
conclude that within this filamentary structure a significant fraction of the
dust mass cycles in and out the small size end of the dust size distribution.Comment: 43 pages, 13 figures, accepted for publication in Ap
Do Proto-Jovian Planets Drive Outflows?
We discuss the possibility that gaseous giant planets drive strong outflows
during early phases of their formation. We consider the range of parameters
appropriate for magneto-centrifugally driven stellar and disk outflow models
and find that if the proto-Jovian planet or accretion disk had a magnetic field
of >~ 10 Gauss and moderate mass inflow rates through the disk of less than
10^-7 M_J/yr that it is possible to drive an outflow. Estimates based both on
scaling from empirical laws observed in proto-stellar outflows and the
magneto-centrigugal disk and stellar+disk wind models suggest that winds with
mass outflow rates of 10^-8 M_J/yr and velocities of order ~ 20 km/s could be
driven from proto-Jovian planets. Prospects for detection and some implications
for the formation of the solar system are briefly discussed.Comment: AAS Latex, accepted for Ap
Lifting the Veil of Dust from NGC 0959: The Importance of a Pixel-Based 2D Extinction Correction
We present the results of a study of the late-type spiral galaxy NGC 0959,
before and after application of the pixel-based dust extinction correction
described in Tamura et al. 2009 (Paper I). Galaxy Evolution Explorer (GALEX)
far-UV (FUV) and near-UV (NUV), ground-based Vatican Advanced Technology
Telescope (VATT) UBVR, and Spitzer/Infrared Array Camera (IRAC) 3.6, 4.5, 5.8,
and 8.0 micron images are studied through pixel Color-Magnitude Diagrams
(pCMDs) and pixel Color-Color Diagrams (pCCDs). We define groups of pixels
based on their distribution in a pCCD of (B - 3.6 micron) versus (FUV - U)
colors after extinction correction. In the same pCCD, we trace their locations
before the extinction correction was applied. This shows that selecting pixel
groups is not meaningful when using colors uncorrected for dust. We also trace
the distribution of the pixel groups on a pixel coordinate map of the galaxy.
We find that the pixel-based (two-dimensional) extinction correction is crucial
to reveal the spatial variations in the dominant stellar population, averaged
over each resolution element. Different types and mixtures of stellar
populations, and galaxy structures such as a previously unrecognized bar,
become readily discernible in the extinction-corrected pCCD and as coherent
spatial structures in the pixel coordinate map.Comment: 10 pages, LaTeX2e requires 'emulateapj.cls', 'graphicx.sty', and
'natbib.sty' (included), 9 postscript figures, 1 table. Accepted for
publication in AJ
The Efficiency of Grain Alignment in Dense Interstellar Clouds: A Reassessment of Constraints from Near Infrared Polarization
A detailed study of interstellar polarization efficiency toward molecular
clouds is used to attempt discrimination between grain alignment mechanisms in
dense regions of the ISM. Background field stars are used to probe polarization
efficiency in quiescent regions of dark clouds, yielding a dependence on visual
extinction well-represented by a power law. No significant change in this
behavior is observed in the transition region between the diffuse outer layers
and dense inner regions of clouds, where icy mantles are formed, and we
conclude that mantle formation has little or no effect on the efficiency of
grain alignment. Young stellar objects generally exhibit greater polarization
efficiency compared with field stars at comparable extinctions, displaying
enhancements by factors of up to 6. Of the proposed alignment mechanisms, that
based on radiative torques appears best able to explain the data. The
attenuated external radiation field accounts for the observed polarization in
quiescent regions, and radiation from the embedded stars themselves may enhance
alignment in the lines of sight to YSOs. Enhancements in polarization
efficiency observed in the ice features toward several YSOs are of greatest
significance, as they demonstrate efficient alignment in cold molecular clouds
associated with star formation
The ortho-to-para ratio of ammonia in the L1157 outflow
We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of
the L1157 outflow by observing the six metastable inversion lines from (J, K) =
(1, 1) to (6, 6). The highly excited (5, 5) and (6, 6) lines were first
detected in the low-mass star forming regions. The rotational temperature
derived from the ratio of four transition lines from (3, 3) to (6, 6) is
130-140 K, suggesting that the blueshifted gas is heated by a factor of ~10 as
compared to the quiescent gas. The ortho-to-para ratio of the NH3 molecules in
the blueshifted gas is estimated to be 1.3--1.7, which is higher than the
statistical equilibrium value. This ratio provides us with evidence that the
NH3 molecules have been evaporated from dust grains with the formation
temperature between 18 and 25 K. It is most likely that the NH3 molecules on
dust grains have been released into the gas phase through the passage of strong
shock waves produced by the outflow. Such a scenario is supported by the fact
that the ammonia abundance in the blueshifted gas is enhanced by a factor of ~5
with respect to the dense quiescent gas.Comment: 16 pages, including 3 PS figures. To appear in the ApJ (Letters).
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Polarization in young open cluster NGC 6823
We present multiwavelength linear polarimetric observations of 104 stars
towards the region of young open cluster NGC 6823. The polarization towards NGC
6823 is dominated by foreground dust grains and we found the evidence for the
presence of several layers of dust towards the line of sight. The first layer
of dust is located approximately within 200 pc towards the cluster, which is
much closer to the Sun than the cluster (~ 2.1 kpc). The radial distribution of
the position angles for the member stars are found to show a systematic change
while the polarization found to reduce towards the outer parts of the cluster
and the average position angle of coronal region of the cluster is very close
to the inclination of the Galactic parallel (~ 32 degree). The size
distribution of the grains within NGC 6823 is similar to those in general
interstellar medium. The patchy distribution of foreground dust grains are
suggested to be mainly responsible for the both differential reddening and
polarization towards NGC 6823. The majority of the observed stars do not show
the evidence of intrinsic polarization in their light.Comment: 16 pages, 6 tables, 11 figures, Accepted for publication in MNRA
Ices in the Quiescent IC 5146 Dense Cloud
This paper presents spectra in the 2 to 20 micron range of quiescent cloud
material located in the IC 5146 cloud complex. The spectra were obtained with
NASA's Infrared Telescope Facility (IRTF) SpeX instrument and the Spitzer Space
Telescope's Infrared Spectrometer. We use these spectra to investigate dust and
ice absorption features in pristine regions of the cloud that are unaltered by
embedded stars. We find that the H2O-ice threshold extinction is 4.03+/-0.05
mag. Once foreground extinction is taken into account, however, the threshold
drops to 3.2 mag, equivalent to that found for the Taurus dark cloud, generally
assumed to be the touchstone quiescent cloud against which all other dense
cloud and embedded young stellar object observations are compared. Substructure
in the trough of the silicate band for two sources is attributed to CH3OH and
NH3 in the ices, present at the ~2% and ~5% levels, respectively, relative to
H2O-ice. The correlation of the silicate feature with the E(J-K) color excess
is found to follow a much shallower slope relative to lines of sight that probe
diffuse clouds, supporting the previous results by Chiar et al. (2007).Comment: 13 pages, 13 figures with multiple parts, accepted for publication in
Astrophysical Journal, Feb. 201
The Stellar Population of the Chamaeleon I Star-Forming Region
I present a new census of the stellar population in the Chamaeleon I
star-forming region. Using optical and near-IR photometry and followup
spectroscopy, I have discovered 50 new members of Chamaeleon I, expanding the
census of known members to 226 objects. Fourteen of these new members have
spectral types later than M6, which doubles the number of known members that
are likely to be substellar. I have estimated extinctions, luminosities, and
effective temperatures for the known members, used these data to construct an
H-R diagram for the cluster, and inferred individual masses and ages with the
theoretical evolutionary models of Baraffe and Chabrier. The distribution of
isochronal ages indicates that star formation began 3-4 and 5-6 Myr ago in the
southern and northern subclusters, respectively, and has continued to the
present time at a declining rate. The IMF in Chamaeleon I reaches a maximum at
a mass of 0.1-0.15 M_sun, and thus closely resembles the IMFs in IC 348 and the
Orion Nebula Cluster. In logarithmic units where the Salpeter slope is 1.35,
the IMF is roughly flat in the substellar regime and shows no indication of
reaching a minimum down to a completeness limit of 0.01 M_sun. The low-mass
stars are more widely distributed than members at other masses in the northern
subcluster, but this is not the case in the southern subcluster. Meanwhile, the
brown dwarfs have the same spatial distribution as the stars out to a radius of
3 deg (8.5 pc) from the center of Chamaeleon I
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