97 research outputs found
The Power of SOFIA/FORCAST in Estimating Internal Luminosities of Low Mass Class 0/I Protostars
With the Stratospheric Observatory for Infrared Astronomy (SOFIA) routinely
operating science flights, we demonstrate that observations with the Faint
Object infraRed CAmera for the SOFIA Telescope (FORCAST) can provide reliable
estimates of the internal luminosities, , of protostars. We have
developed a technique to estimate using a pair of FORCAST
filters: one "short-wavelength" filter centered within 19.7-25.3 m, and
one "long-wavelength" filter within 31.5-37.1 m. These
estimates are reliable to within 30-40% for 67% of protostars and to within a
factor of 2.3-2.6 for 99% of protostars. The filter pair comprised of
F25.3m and F37.1m achieves the best sensitivity and most constrained
results. We evaluate several assumptions that could lead to systematic
uncertainties. The OH5 dust opacity matches observational constraints for
protostellar environments best, though not perfectly; we find that any improved
dust model will have a small impact of 5-10% on the estimates.
For protostellar envelopes, the TSC84 model yields masses that are twice those
of the Ulrich model, but we conclude this mass difference does not
significantly impact results at the mid-infrared wavelengths probed by FORCAST.
Thus, FORCAST is a powerful instrument for luminosity studies targeting newly
discovered protostars or suspected protostars lacking detections longward of 24
m. Furthermore, with its dynamic range and greater angular resolution,
FORCAST may be used to characterize protostars that were either saturated or
merged with other sources in previous surveys using the Spitzer Space Telescope
or Herschel Space Observatory.Comment: 17 pages, 9 figures. Accepted for publication in Ap
The Dynamical State of Barnard 68: A Thermally Supported, Pulsating Dark Cloud
We report sensitive, high resolution molecular-line observations of the dark
cloud Barnard 68 obtained with the IRAM 30-m telescope. We analyze
spectral-line observations of C18O, CS(2--1), C34S(2--1), and N2H+(1--0) in
order to investigate the kinematics and dynamical state of the cloud. We find
extremely narrow linewidths in the central regions of the cloud. These narrow
lines are consistent with thermally broadened profiles for the measured gas
temperature of 10.5 K. We determine the thermal pressure to be a factor 4 -- 5
times greater than the non-thermal (turbulent) pressure in the central regions
of the cloud, indicating that thermal pressure is the primary source of support
against gravity in this cloud. This confirms the inference of a thermally
supported cloud drawn previously from deep infrared extinction measurements.
The rotational kinetic energy is found to be only a few percent of the
gravitational potential energy, indicating that the contribution of rotation to
the overall stability of the cloud is insignificant. Finally, our observations
show that CS line is optically thick and self-reversed across nearly the entire
projected surface of the cloud. The shapes of the self-reversed profiles are
asymmetric and are found to vary across the cloud in such a manner that the
presence of both inward and outward motions are observed within the cloud.
Moreover, these motions appear to be globally organized in a clear and
systematic alternating spatial pattern which is suggestive of a small
amplitude, non-radial oscillation or pulsation of the outer layers of the cloud
about an equilibrium configuration.Comment: To appear in the Astrophysical Journal; 23 pages, 8 figures;
Manuscript and higher resolution images can be obtained at
http://cfa-www.harvard.edu/~ebergin/pubs_html/b68_vel.htm
The Spitzer c2d Survey of Nearby Dense Cores. IX. Discovery of a Very Low Luminosity Object Driving a Molecular Outflow in the Dense Core L673-7
We present new infrared, submillimeter, and millimeter observations of the
dense core L673-7 and report the discovery of a low-luminosity, embedded Class
0 protostar driving a molecular outflow. L673-7 is seen in absorption against
the mid-infrared background in 5.8, 8, and 24 micron Spitzer images, allowing
for a derivation of the column density profile and total enclosed mass of
L673-7, independent of dust temperature assumptions. Estimates of the core mass
from these absorption profiles range from 0.2-4.5 solar masses. Millimeter
continuum emission indicates a mass of about 2 solar masses, both from a direct
calculation assuming isothermal dust and from dust radiative transfer models
constrained by the millimeter observations. We use dust radiative transfer
models to constrain the internal luminosity of L673-7, defined to be the
luminosity of the central source and excluding the luminosity from external
heating, to be 0.01-0.045 solar luminosities, with 0.04 solar luminosities the
most likely value. L673-7 is thus classified as a very low luminosity object
(VeLLO), and is among the lowest luminosity VeLLOs yet studied. We calculate
the kinematic and dynamic properties of the molecular outflow in the standard
manner, and we show that the expected accretion luminosity based on these
outflow properties is greater than or equal to 0.36 solar luminosities. The
discrepancy between this expected accretion luminosity and the internal
luminosity derived from dust radiative transfer models indicates that the
current accretion rate is much lower than the average rate over the lifetime of
the outflow. Although the protostar embedded within L673-7 is consistent with
currently being substellar, it is unlikely to remain as such given the
substantial mass reservoir remaining in the core.Comment: 19 pages, 14 figures. Accepted by Ap
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The Spitzer c2d Survey Of Nearby Dense Cores. X. Star Formation In L673 And Cb188
L673 and CB188 are two low-mass clouds isolated from large star-forming regions that were observed as part of the Spitzer Legacy Project "From Molecular Clouds to Planet Forming disks" (c2d). We identified and characterized all the young stellar objects (YSOs) of these two regions and modeled their spectral energy distributions (SEDs) to examine whether their physical properties are consistent with values predicted from the theoretical models and with the YSO properties in the c2d survey of larger clouds. Overall, 30 YSO candidates were identified by the c2d photometric criteria, 27 in L673 and 3 in CB188. We confirm the YSO nature of 29 of them and remove a false Class III candidate in L673. We further present the discovery of two new YSO candidates, one Class 0 and another possible Class I candidate in L673, therefore bringing the total number of YSO candidates to 31. Multiple sites of star formation are present within L673, closely resembling other well-studied c2d clouds containing small groups such as B59 and L1251B, whereas CB188 seems to consist of only one isolated globule-like core. We measure a star formation efficiency (SFE) of 4.6%, which resembles the SFE of the larger c2d clouds. From the SED modeling of our YSO sample we obtain envelope masses for Class I and Flat spectrum sources of 0.01-1.0 M-circle dot. The majority of Class II YSOs show disk accretion rates from 3.3 x 10(-10) to 3 x 10(-8) M-circle dot yr(-1) and disk masses that peak at 10(-4) to 10(-3) M-circle dot. Finally, we examined the possibility of thermal fragmentation in L673 as the main star-forming process. We find that the mean density of the regions where significant YSO clustering occurs is of the order of similar to 10(5) cm(-3) using 850 mu m observations and measure a Jeans Length that is greater than the near-neighbor YSO separations by approximately a factor of 3-4. We therefore suggest that other processes, such as turbulence and shock waves, may have had a significant effect on the cloud's filamentary structure and YSO clustering.University of SouthamptonNASA 1279198, 1288806, 1365763Jet Propulsion Laboratory, California Institute of TechnologyAstronom
Observational Constraints on Submillimeter Dust Opacity
Infrared extinction maps and submillimeter dust continuum maps are powerful probes of the density structure in the envelope of star-forming cores. We make a direct comparison between infrared and submillimeter dust continuum observations of the low-mass Class 0 core, B335, to constrain the ratio of submillimeter to infrared opacity (Îș_(smm)/Îș_(ir)) and the submillimeter opacity power-law index (Îș â λâÎČ). Using the average value of theoretical dust opacity models at 2.2 ÎŒm, we constrain the dust opacity at 850 and 450 ÎŒm. Using new dust continuum models based upon the broken power-law density structure derived from interferometric observations of B335 and the infall model derived from molecular line observations of B335, we find that the opacity ratios are ^Îș_(850)_Îș_(2.2) = (3.21 - 4.80)^(+0.44)_(-0.30) x 10^(-4) ^Îș_(450)_Îș(2.0) = (12.8-24.8)^(+2.4)_(-1.3) x 10^(-4) with a submillimeter opacity power-law index of ÎČ_(smm) = (2.18-2.58)^(+0.30)_(â0.30). The range of quoted values is determined from the uncertainty in the physical model for B335. For an average 2.2 ÎŒm opacity of 3800 ± 700 cm^2 g^(â1), we find a dust opacity at 850 and 450 ÎŒm of Îș_(850) = (1.18-1.77)^9+0.36)_(â0.24) and Îș_(450) = (4.72-9.13)^(+1.9)_(â0.98) cm^2 g^(â1) of dust. These opacities are from (0.65-0.97)Îș^(OH5)_(850) of the widely used theoretical opacities of Ossenkopf and Henning for coagulated ice grains with thin mantles at 850 ÎŒm
The Spitzer c2d Survey of Nearby Dense Cores. V. Discovery of a VeLLO in the "Starless" Dense Core L328
This paper reports the discovery of a Very Low Luminosity Object (VeLLO) in
the "starless" dense core L328, using the Spitzer Space Telescope and ground
based observations from near-infrared to millimeter wavelengths. The Spitzer 8
micron image indicates that L328 consists of three subcores of which the
smallest one may harbor a source, L328-IRS while two other subcores remain
starless. L328-IRS is a Class 0 protostar according to its bolometric
temperature (44 K) and the high fraction ~72 % of its luminosity emitted at
sub-millimeter wavelengths. Its inferred "internal luminosity" (0.04 - 0.06
Lsun) using a radiative transfer model under the most plausible assumption of
its distance as 200 pc is much fainter than for a typical protostar, and even
fainter than other VeLLOs studied previously. Note, however, that its inferred
luminosity may be uncertain by a factor of 2-3 if we consider two extreme
values of the distance of L328-IRS (125 or 310 pc). Low angular resolution
observations of CO do not show any clear evidence of a molecular outflow
activity. But broad line widths toward L328, and Spitzer and near-infrared
images showing nebulosity possibly tracing an outflow cavity, strongly suggest
the existence of outflow activity. Provided that an envelope of at most ~0.1
Msunis the only mass accretion reservoir for L328-IRS, and the star formation
efficiency is close to the canonical value ~30%, L328-IRS has not yet accreted
more than 0.05 Msun. At the assumed distance of 200 pc, L328-IRS is destined to
be a brown dwarf.Comment: 29 pages, 8 figures, 1 table, to be published in Astrophysical
Journa
The Spitzer c2d Survey Of Nearby Dense Cores. XI. Infrared And Submillimeter Observations Of CB130
We present new observations of the CB130 region composed of three separate cores. Using the Spitzer Space Telescope, we detected a Class 0 and a Class II object in one of these, CB130-1. The observed photometric data from Spitzer and ground-based telescopes are used to establish the physical parameters of the Class 0 object. Spectral energy distribution fitting with a radiative transfer model shows that the luminosity of the Class 0 object is 0.14-0.16 L-circle dot, which is low for a protostellar object. In order to constrain the chemical characteristics of the core having the low-luminosity object, we compare our molecular line observations to models of lines including abundance variations. We tested both ad hoc step function abundance models and a series of self-consistent chemical evolution models. In the chemical evolution models, we consider a continuous accretion model and an episodic accretion model to explore how variable luminosity affects the chemistry. The step function abundance models can match observed lines reasonably well. The best-fitting chemical evolution model requires episodic accretion and the formation of CO2 ice from CO ice during the low-luminosity periods. This process removes C from the gas phase, providing a much improved fit to the observed gas-phase molecular lines and the CO2 ice absorption feature. Based on the chemical model result, the low luminosity of CB130-1 is explained better as a quiescent stage between episodic accretion bursts rather than being at the first hydrostatic core stage.NASA 1224608, 1288664, 1407, NNX07AJ72G, 1279198, 1288806, 1342425NSF AST-0607793, AST-0708158Korea government (MEST) 2009-0062866Ministry of Education, Science and Technology 2010-0008704Astronom
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed With IRAC and MIPS
We present c2d Spitzer/IRAC observations of the Lupus I, III and IV dark
clouds and discuss them in combination with optical and near-infrared and c2d
MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times
larger than the previous one. It is dominated by low- and very-low mass stars
and it is complete down to M 0.1M. We find 30-40 % binaries
with separations between 100 to 2000 AU with no apparent effect in the disk
properties of the members. A large majority of the objects are Class II or
Class III objects, with only 20 (12%) of Class I or Flat spectrum sources. The
disk sample is complete down to ``debris''-like systems in stars as small as M
0.2 M and includes sub-stellar objects with larger IR
excesses. The disk fraction in Lupus is 70 -- 80%, consistent with an age of 1
-- 2 Myr. However, the young population contains 20% optically thick accretion
disks and 40% relatively less flared disks. A growing variety of inner disk
structures is found for larger inner disk clearings for equal disk masses.
Lupus III is the most centrally populated and rich, followed by Lupus I with a
filamentary structure and by Lupus IV, where a very high density core with
little star-formation activity has been found. We estimate star formation rates
in Lupus of 2 -- 10 M Myr and star formation efficiencies of a
few percent, apparently correlated with the associated cloud masses.Comment: Accepted for publication in the ApJS. Contains 101 pages, 23 figures,
and 13 tables. A version with full resolution figures can be found at
http://peggysue.as.utexas.edu/SIRTF/PAPERS/pap102.pub.pd
The Spitzer c2d survey of large, nearby, interstellar clouds. X. The Chamaeleon II pre-main-sequence population as observed with IRAC and MIPS
We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg^(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 Mâ). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 Mâ pc^(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8" < Ξ < 6.0" (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Ît ~ 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 Mâ into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered
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