50 research outputs found
Warm gas towards young stellar objects in Corona Australis - Herschel/PACS observations from the DIGIT key programme
The effects of external irradiation on the chemistry and physics in the
protostellar envelope around low-mass young stellar objects are poorly
understood. The Corona Australis star-forming region contains the R CrA dark
cloud, comprising several low-mass protostellar cores irradiated by an
intermediate-mass young star. We study the effects on the warm gas and dust in
a group of low-mass young stellar objects from the irradiation by the young
luminous Herbig Be star R CrA. Herschel/PACS far-infrared datacubes of two
low-mass star-forming regions in the R CrA dark cloud are presented. The
distribution of CO, OH, H2O, [C II], [O I], and continuum emission is
investigated. We have developed a deconvolution algorithm which we use to
deconvolve the maps, separating the point-source emission from the extended
emission. We also construct rotational diagrams of the molecular species. By
deconvolution of the Herschel data, we find large-scale (several thousand AU)
dust continuum and spectral line emission not associated with the point
sources. Similar rotational temperatures are found for the warm CO (
K), hot CO ( K), OH ( K), and H2O ( K) emission,
respectively, in the point sources and the extended emission. The rotational
temperatures are also similar to what is found in other more isolated cores.
The extended dust continuum emission is found in two ridges similar in extent
and temperature to molecular mm emission, indicative of external heating from
the Herbig Be star R CrA. Our results show that a nearby luminous star does not
increase the molecular excitation temperatures in the warm gas around a young
stellar object (YSO). However, the emission from photodissociation products of
H2O, such as OH and O, is enhanced in the warm gas associated with these
protostars and their surroundings compared to similar objects not suffering
from external irradiation.Comment: 37 pages, accepted for publication in A&
Velocity-resolved high-J CO emission from massive star-forming clumps
(Abridged) Context. Massive star formation is associated with energetic
processes, which result in significant gas cooling via far-infrared (IR) lines.
Velocity-resolved observations can constrain the kinematics of the gas,
allowing the identification of the physical mechanisms responsible for gas
heating. Aims. Our aim is to quantify far-infrared CO line emission toward
high-mass star-forming regions, identify the high-velocity gas component
associated with outflows, and estimate the physical conditions required for the
excitation of the observed lines. Methods. Velocity-resolved SOFIA/GREAT
spectra of 13 high-mass star forming clumps of various luminosities and
evolutionary stages are studied using CO 11-10 and 16-15 lines. Results. All
targets show strong high-J CO emission in the far-IR, characterized by broad
line wings associated with outflows, thereby significantly increasing the
sample of sources with velocity-resolved high-J CO spectra. The contribution of
the emission in the line wings does not correlate with the envelope mass or
evolutionary stage. Gas rotational temperatures cover a narrow range of 120-220
K for the line wings. The non-LTE radiative transfer models indicate gas
densities of 1e5-1e7 cm-3 and N(CO) of 1e17- 1e18 cm-2, similar to physical
conditions in deeply-embedded low- and high-mass protostars. The
velocity-integrated CO line fluxes correlate with the bolometric luminosity
over 7 orders of magnitude including data on the low-mass protostars,
suggesting similar processes are responsible for the high-J CO excitation over
a significant range of physical scales. Conclusions. Velocity-resolved line
profiles allow the detection of outflows toward massive star-forming clumps
spanning a broad range of evolutionary stages. The lack of clear evolutionary
trends suggest that mass accretion and ejection prevail during the entire
lifetime of star-forming clumps.Comment: 21 pages, 19 figures, accepted to A&
Identifying Young Stellar Objects in the Outer Galaxy: l = 224 deg Region in Canis Major
We study a very young star-forming region in the outer Galaxy that is the
most concentrated source of outflows in the Spitzer Space Telescope GLIMPSE360
survey. This region, dubbed CMa-l224, is located in the Canis Major OB1
association. CMa-l224 is relatively faint in the mid-infrared, but it shines
brightly at the far-infrared wavelengths as revealed by the Herschel Space
Observatory data from the Hi-GAL survey. Using the 3.6 and 4.5 m data from
the Spitzer/GLIMPSE360 survey, combined with the JHK 2MASS and the 70-500
m Herschel/Hi-GAL data, we develop a young stellar object (YSO) selection
criteria based on color-color cuts and fitting of the YSO candidates' spectral
energy distributions with YSO 2D radiative transfer models. We identify 293 YSO
candidates and estimate physical parameters for 210 sources well-fit with YSO
models. We select an additional 47 sources with GLIMPSE360-only photometry as
`possible YSO candidates'. The vast majority of these sources are associated
with high H column density regions and are good targets for follow-up
studies. The distribution of YSO candidates at different evolutionary stages
with respect to Herschel filaments supports the idea that stars are formed in
the filaments and become more dispersed with time. Both the supernova-induced
and spontaneous star formation scenarios are plausible in the environmental
context of CMa-l224. However, our results indicate that a spontaneous
gravitational collapse of filaments is a more likely scenario. The methods
developed for CMa-l224 can be used for larger regions in the Galactic plane
where the same set of photometry is available.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series; 54 pages including appendice
CO in Protostars (COPS): -SPIRE Spectroscopy of Embedded Protostars
We present full spectral scans from 200-670m of 26 Class 0+I
protostellar sources, obtained with -SPIRE, as part of the
"COPS-SPIRE" Open Time program, complementary to the DIGIT and WISH Key
programs. Based on our nearly continuous, line-free spectra from 200-670
m, the calculated bolometric luminosities () increase by 50%
on average, and the bolometric temperatures () decrease by 10% on
average, in comparison with the measurements without Herschel. Fifteen
protostars have the same Class using and /. We identify rotational transitions of CO lines from J=4-3 to J=13-12,
along with emission lines of CO, HCO, HO, and [CI]. The ratios
of CO to CO indicate that CO emission remains optically
thick for < 13. We fit up to four components of temperature from
the rotational diagram with flexible break points to separate the components.
The distribution of rotational temperatures shows a primary population around
100 K with a secondary population at 350 K. We quantify the correlations
of each line pair found in our dataset, and find the strength of correlation of
CO lines decreases as the difference between -level between two CO lines
increases. The multiple origins of CO emission previously revealed by
velocity-resolved profiles are consistent with this smooth distribution if each
physical component contributes to a wide range of CO lines with significant
overlap in the CO ladder. We investigate the spatial extent of CO emission and
find that the morphology is more centrally peaked and less bipolar at high-
lines. We find the CO emission observed with SPIRE related to outflows, which
consists two components, the entrained gas and shocked gas, as revealed by our
rotational diagram analysis as well as the studies with velocity-resolved CO
emission.Comment: 50 pages, 18 figures, accepted to ApJS. Revised for Table 6 and
Figure
APEX-CHAMP+ high-J CO observations of low-mass young stellar objects: IV. Mechanical and radiative feedback
During the embedded stage of star formation, bipolar molecular outflows and
UV radiation from the protostar are important feedback processes. Our aim is to
quantify the feedback, mechanical and radiative, for a large sample of low-mass
sources. The outflow activity is compared to radiative feedback in the form of
UV heating by the accreting protostar to search for correlations and
evolutionary trends. Large-scale maps of 26 young stellar objects, which are
part of the Herschel WISH key program are obtained using the CHAMP+ instrument
on the APEX (12CO and 13CO 6-5), and the HARP-B instrument on the JCMT (12CO
and 13CO 3-2). Maps are used to determine outflow parameters and envelope
models are used to quantify the amount of UV-heated gas and its temperature
from 13CO 6-5 observations. All sources in our sample show outflow activity and
the outflow force, F_CO, is larger for Class 0 sources than for Class I
sources, even if their luminosities are comparable. The outflowing gas
typically extends to much greater distances than the power-law envelope and
therefore influences the surrounding cloud material directly. Comparison of the
CO 6-5 results with Herschel-HIFI H2O and PACS high-J CO lines, both tracing
currently shocked gas, shows that the two components are linked, even though
the transitions do not probe the same gas. The link does not extend down to CO
3-2. The conclusion is that CO 6-5 depends on the shock characteristics
(density and velocity), whereas CO 3-2 is more sensitive to conditions in the
surrounding environment (density). The radiative feedback is responsible for
increasing the gas temperature by a factor of two, up to 30-50 K, on scales of
a few thousand AU, particularly along the direction of the outflow. The mass of
the UV heated gas exceeds the mass contained in the entrained outflow in the
inner ~3000 AU and is therefore at least as important on small scales.Comment: 30 pages with Appendix, Accepted by Astronomy & Astrophysic
Investigating the Impact of Metallicity on Star Formation in the Outer Galaxy. I. VLT/KMOS Survey of Young Stellar Objects in Canis Major
The effects of metallicity on the evolution of protoplanetary disks may be
studied in the outer Galaxy where the metallicity is lower than in the solar
neighbourhood. We present the VLT/KMOS integral field spectroscopy in the
near-infrared of 120 candidate young stellar objects (YSOs) in the
CMa-224 star-forming region located at a Galactocentric distance of 9.1
kpc. We characterise the YSO accretion luminosities and accretion rates using
the hydrogen Br emission and find the median accretion luminosity of
. Based on the measured
accretion luminosities, we investigate the hypothesis of star formation history
in the CMa-224. Their median values suggest that Cluster C, where most of
YSO candidates have been identified, might be the most evolved part of the
region. The accretion luminosities are similar to those observed toward
low-mass YSOs in the Perseus and Orion molecular clouds, and do not reveal the
impact of lower metallicity. Similar studies in other outer Galaxy clouds
covering a wide range of metallicities are critical to gain a complete picture
of star formation in the Galaxy.Comment: Accepted for publication in APJS, 51 pages, 37 figures, 6 table
Water in star-forming regions with Herschel: highly excited molecular emission from the NGC 1333 IRAS 4B outflow
During the embedded phase of pre-main sequence stellar evolution, a disk
forms from the dense envelope while an accretion-driven outflow carves out a
cavity within the envelope. Highly excited H2O emission in spatially unresolved
Spitzer/IRS spectra of a low-mass Class 0 object, NGC 1333 IRAS 4B, has
previously been attributed to the envelope-disk accretion shock but could
instead be produced in an outflow. As part of the survey of low-mass sources in
the Water in Star Forming Regions with Herschel (WISH-LM) program, we used
Herschel/PACS to obtain a far-IR spectrum and several Nyquist-sampled spectral
images with to determine the origin of excited H2O emission from NGC 1333 IRAS
4B. The spectrum has high signal-to-noise in a rich forest of H2O, CO, and OH
lines, providing a near-complete census of far-IR molecular emission from a
Class 0 protostar. The excitation diagrams for the three molecules all require
fits with two excitation temperatures, indicating the presence of two physical
components. The highly excited component of H2O emission is characterized by
subthermal excitation of 1500 K gas with a density of 10^5 - 10^7 cm-3,
conditions that also reproduce the mid-IR H2O emission detected by Spitzer. On
the other hand, a high density, low temperature gas can reproduce the H2O
spectrum observed by Spitzer but underpredicts the H2O lines seen by Herschel.
Nyquist-sampled spectral maps of several lines show two spatial components of
H2O emission, one centered at 1200 AU south of the central source at the
position of the blueshifted outflow lobe and a second centered on-source. Both
spatial components of the far-IR H2O emission are consistent with emission from
the outflow. The gas cooling from the IRAS 4B envelope cavity walls is
dominated by far-IR H2O emission, in contrast to stronger [O I] and CO cooling
from more evolved protostars. [one sentence truncated]Comment: 24 total pages; accepted by A&
The Detection of Deuterated Water in the Large Magellanic Cloud with ALMA
We report the first detection of deuterated water (HDO) toward an
extragalactic hot core. The HDO 2-2 line has been detected toward
hot cores N105-2A and 2B in the N105 star-forming region in the low-metallicity
Large Magellanic Cloud (LMC) dwarf galaxy with the Atacama Large
Millimeter/submillimeter Array (ALMA). We have compared the HDO line luminosity
() measured toward the LMC hot cores to those observed toward a
sample of seventeen Galactic hot cores covering three orders of magnitude in
, four orders of magnitude in bolometric luminosity (), and a wide range of Galactocentric distances (thus metallicities). The
observed values of for the LMC hot cores fit very well into the
trends with and metallicity observed toward the
Galactic hot cores. We have found that seems to be largely
dependent on the source luminosity, but metallicity also plays a role. We
provide a rough estimate of the HO column density and abundance ranges
toward the LMC hot cores by assuming that HDO/HO toward the LMC hot cores
is the same as that observed in the Milky Way; the estimated ranges are
systematically lower than Galactic values. The spatial distribution and
velocity structure of the HDO emission in N105-2A is consistent with HDO being
the product of the low-temperature dust grain chemistry. Our results are in
agreement with the astrochemical model predictions that HDO is abundant
regardless of the extragalactic environment and should be detectable with ALMA
in external galaxies.Comment: 21 pages, 2 tables, 9 figures (including appendices); Accepted for
publication in the Astrophysical Journa