123 research outputs found
The mid-infrared extinction law in the darkest cores of the Pipe Nebula
Context. The properties of dust grains, in particular their size
distribution, are expected to differ from the interstellar medium to the
high-density regions within molecular clouds. Aims. We measure the mid-infrared
extinction law produced by dense material in molecular cloud cores. Since the
extinction at these wavelengths is caused by dust, the extinction law in cores
should depart from that found in low-density environments if the dust grains
have different properties. Methods. We use the unbiased LINES method to measure
the slope of the reddening vectors in color-color diagrams. We derive the
mid-infrared extinction law toward the dense cores B59 and FeSt 1-457 in the
Pipe Nebula over a range of visual extinction between 10 and 50 magnitudes,
using a combination of Spitzer/IRAC, and ESO NTT/VLT data. Results. The
mid-infrared extinction law in both cores departs significantly from a
power-law between 3.6 and 8 micron, suggesting that these cores contain dust
with a considerable fraction of large dust grains. We find no evidence for a
dependence of the extinction law with column density up to 50 magnitudes of
visual extinction in these cores, and no evidence for a variation between our
result and those for other clouds at lower column densities reported elsewhere
in the literature. This suggests that either large grains are present even in
low column density regions, or that the existing dust models need to be revised
at mid-infrared wavelengths. We find a small but significant difference in the
extinction law of the two cores, that we tentatively associate with the onset
of star formation in B59.Comment: 8 pages, 6 figures. Accepted to A&
The Age, Stellar Content and Star Formation Timescale of the B59 Dense Core
We have used moderate resolution, near-infrared spectra from the SpeX
spectrograph on the NASA Infrared Telescope facility to characterize the
stellar content of Barnard 59 (B59), the most active star-forming core in the
Pipe Nebula. Measuring luminosity and temperature sensitive features in the
spectra of 20 candidate YSOs, we identified likely background giant stars and
measured each star's spectral type, extinction, and NIR continuum excess. We
find that B59 is composed of late type (K4-M6) low-mass (0.9--0.1 M_sun) YSOs
whose median stellar age is comparable to, if not slightly older than, that of
YSOs within the Rho Oph, Taurus, and Chameleon star forming regions. Deriving
absolute age estimates from pre-main sequence models computed by D'Antona et
al., and accounting only for statistical uncertainties, we measure B59's median
stellar age to be 2.6+/-0.8 Myrs. Including potential systematic effects
increases the error budget for B59's median (DM98) stellar age to 2.6+4.1/-2.6
Myrs. We also find that the relative age orderings implied by pre-main sequence
evolutionary tracks depend on the range of stellar masses sampled, as model
isochrones possess significantly different mass dependencies. The maximum
likelihood median stellar age we measure for B59, and the region's observed gas
properties, suggest that the B59 dense core has been stable against global
collapse for roughly 6 dynamical timescales, and is actively forming stars with
a star formation efficiency per dynamical time of ~6%. This maximum likelihood
value agrees well with recent star formation simulations that incorporate
various forms of support against collapse, such as sub-critical magnetic
fields, outflows, and radiative feedback from protostellar heating. [abridged]Comment: 20 pages, 12 figures, accepted for publication in the Astrophysical
Journal; updated to amend acknowledgment
The extinction law from photometric data: linear regression methods
Context. The properties of dust grains, in particular their size
distribution, are expected to differ from the interstellar medium to the
high-density regions within molecular clouds. Since the extinction at
near-infrared wavelengths is caused by dust, the extinction law in cores should
depart from that found in low-density environments if the dust grains have
different properties. Aims. We explore methods to measure the near-infrared
extinction law produced by dense material in molecular cloud cores from
photometric data. Methods. Using controlled sets of synthetic and
semi-synthetic data, we test several methods for linear regression applied to
the specific problem of deriving the extinction law from photometric data. We
cover the parameter space appropriate to this type of observations. Results. We
find that many of the common linear-regression methods produce biased results
when applied to the extinction law from photometric colors. We propose and
validate a new method, LinES, as the most reliable for this effect. We explore
the use of this method to detect whether or not the extinction law of a given
reddened population has a break at some value of extinction.Comment: 15 pages, 18 figures, accepted to A&A, in pres
The Relationship Between the Dust and Gas-Phase CO Across the California Molecular Cloud
S. Kong, et al., âThe Relationship Between the Dust and Gas-Phase CO Across the California Molecular Cloudâ, The Astrophysical Journal, Vol. 805(1), May 2015. © 2015. The American Astronomical Society.A deep, wide-field, near-infrared imaging survey was used to construct an extinction map of the southeastern part of the California Molecular Cloud (CMC) with 0.5 arc min resolution. The same region was also surveyed in the CO(2-1), CO(2-1), CO(2-1) emission lines at the same angular resolution. Strong spatial variations in the abundances of CO and CO were found to be correlated with variations in gas temperature, consistent with temperature dependent CO depletion/desorption on dust grains. The CO to CO abundance ratio was found to increase with decreasing extinction, suggesting selective photodissociation of CO by the ambient UV radiation field. The cloud averaged X-factor is found to be 2.53 10 , somewhat higher than the Milky Way average. On sub-parsec scales we find no single empirical value of the X-factor that can characterize the molecular gas in cold (T 15 K) regions, with X A for A 3 magnitudes. However in regions containing relatively hot (T 25 K) gas we find a clear correlation between W(CO) and A over a large (3 A 25 mag) extinction range. This suggests a constant X 1.5 10 for the hot gas, a lower value than either the average for the CMC or Milky Way. We find a correlation between X and T with X T suggesting that the global X-factor of a cloud may depend on the relative amounts of hot gas within it.Peer reviewe
The star-forming cores in the centre of the Trifid nebula (M 20): from Herschel to the near-infrared
A new detailed infrared (IR) study of eight star-forming dense condensations (TCs) in M 20, the Trifid nebula, is presented. The aim is to determine the physical properties of the dust in such globules and establish the presence and properties of their embedded protostellar and/or young stellar population. For this, we analysed new Herschel far-IR and Calar Alto near-IR images of the region, combined with Spitzer Infrared Array Camera (Spitzer/IRAC) archival observations. We confirm the presence of several young stellar objects (YSOs), most with mid-IR colours of Class II sources in all but one of the observed cores. Five TCs are dominated in the far-IR by Class I sources with bolometric luminosities between 100 and 500 Lâ. We report the discovery of a possible counterjet to HH 399 and its protostellar engine inside the photodissociation region TC2, as well as a bipolar outflow system, signposted by symmetric H2 emission knots, embedded in TC3. The present results are compatible with previous suggestions that star formation has been active in the region for some 3 Ă 105 yr, and that the most recent events in some of these TCs may have been triggered by the expansion of the H II region. We also obtained a revised value for the distance to M 20 of 2.0 ± 0.1 kpc
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