Context.
Infrared dark clouds (IRDCs) are useful target sources for the studies of molecular cloud substructure evolution and early stages of star formation. Determining the chemical composition of ...IRDCs helps to constrain the initial conditions and timescales (via chemical clocks) of star formation in these often filamentary, dense interstellar clouds.
Aims.
We aim to determine the fractional abundances of multiple different molecular species in the filamentary IRDC G304.74+01.32, nicknamed the Seahorse IRDC, and to search for relationships between the abundances and potential evolutionary trends.
Methods.
We used the Atacama Pathfinder EXperiment (APEX) telescope to observe spectral lines occurring at about 170 GHz frequency towards 14 positions along the full extent of the Seahorse filament. The sample is composed of five clumps that appear dark in the mid-IR, eight clumps that are associated with mid-IR sources, and one clump that is already hosting an H
II
region and is, hence, likely to be in the most advanced stage of evolution of all the target sources. We also employed our previous 870
μ
m dust continuum imaging data of the Seahorse.
Results.
Six spectral line transitions were detected (≥3
σ
) altogether, namely, SO(
N
J
= 4
4
−3
3
), H
13
CN(
J
= 2−1), H
13
CO
+
(
J
= 2−1), SiO(
J
= 4−3), HN
13
C(
J
= 2−1), and C
2
H(
N
= 2−1). While SO, H
13
CO
+
, and HN
13
C were detected in every source, the detection rates for C
2
H and H
13
CN were 92.9 and 85.7%, respectively. Only one source (SMM 3) showed detectable SiO emission (7.1% detection rate). Three clumps (SMM 5, 6, and 7) showed the SO, H
13
CN, H
13
CO
+
, HN
13
C, and C
2
H lines in absorption. Of the detected species, C
2
H was found to be the most abundant one with respect to H
2
(a few times 10
−9
on average), while HN
13
C was found to be the least abundant species (a few times 10
−11
). We found three positive correlations among the derived molecular abundances, of which those between C
2
H and HN
13
C and HN
13
C and H
13
CO
+
are the most significant (correlation coefficient
r
≃ 0.9). The statistically most significant evolutionary trends we uncovered are the drops in the C
2
H abundance and in the HN
13
C∕H
13
CN ratio as the clump evolves from an IR dark stage to an IR bright stage and then to an H
II
region.
Conclusions.
The absorption lines detected towards SMM 6 and SMM 7 could arise from continuum radiation from an embedded young stellar object and an extragalactic object seen along the line of sight. However, the cause of absorption lines in the IR dark clump SMM 5 remains unclear. The correlations we found between the different molecular abundances can be understood as arising from the gas-phase electron (ionisation degree) and atomic carbon abundances. With the exception of H
13
CN and H
13
CO
+
, the fractional abundances of the detected molecules in the Seahorse are relatively low compared to those in other IRDC sources. The C
2
H evolutionary indicator we found is in agreement with previous studies, and can be explained by the conversion of C
2
H to other species (e.g. CO) when the clump temperature rises, especially after the ignition of a hot molecular core in the clump. The decrease of HN
13
C∕H
13
CN as the clump evolves is also likely to reflect the increase in the clump temperature, which leads to an enhanced formation of HCN and its
13
C isotopologue. Both single-dish and high-resolution interferometric imaging of molecular line emission (or absorption) of the Seahorse filament are required to understand the large-scale spatial distribution of the gas and to search for possible hot, high-mass star-forming cores in the cloud.
Context. Infrared dark clouds (IRDCs) provide a useful testbed in which to investigate the genuine initial conditions and early stages of massive-star formation. Aims. We attempt to characterise the ...chemical properties of a sample of 35 massive clumps of IRDCs through multi-molecular line observations. We also search for possible evolutionary trends among the derived chemical parameters. Methods. The clumps are studied using the MALT90 (Millimetre Astronomy Legacy Team 90 GHz) line survey data obtained with the Mopra 22 m telescope. The survey covers 16 different transitions near 90 GHz. The spectral-line data are used in concert with our previous LABOCA (Large APEX BOlometer CAmera) 870 μm dust emission data. Results. Eleven MALT90 transitions are detected towards the clumps at least at the 3σ level. Most of the detected species (SiO, C2H, HNCO, HCN, HCO+, HNC, HC3N, and N2H+) show spatially extended emission towards many of the sources. Most of the fractional abundances of the molecules with respect to H2 are found to be comparable to those determined in other recent similar studies of IRDC clumps. We found that the abundances of SiO, HNCO, and HCO+ are higher in IR-bright clumps than in IR-dark sources, reflecting a possible evolutionary trend. A hint of this trend is also seen for HNC and HC3N. An opposite trend is seen for the C2H and N2H+ abundances. Moreover, a positive correlation is found between the abundances of HCO+ and HNC, and between those of HNC and HCN. The HCN and HNC abundances also appear to increase as a function of the N2H+ abundance. The HNC/HCN and N2H+/HNC abundance ratios are derived to be near unity on average, while that of HC3N/HCN is ~10%. The N2H+/HNC ratio appears to increase as the clump evolves, while the HNC/HCO+ ratio shows the opposite behaviour. Conclusions. The detected SiO emission is probably caused by shocks driven by outflows in most cases, although shocks resulting from the cloud formation process could also play a role. Shock-origin for the HNCO, HC3N, and CH3CN emission is also plausible. The average HNC/HCN ratio is in good agreement with those seen in other IRDCs, but gas temperature measurements would be neeeded to study its temperature dependence. Our results support the finding that C2H can trace the cold gas, and not just the photodissociation regions. The HC3N/HCN ratio appears to be comparable to the values seen in other types of objects, such as T Tauri disks and comets.
Context.
Infrared dark clouds (IRDCs) can be the birth sites of high-mass stars, and hence determining the physical properties of dense cores in IRDCs is useful to constrain the initial conditions ...and theoretical models of high-mass star formation.
Aims.
We aim to determine the physical properties of dense cores in the filamentary Seahorse IRDC G304.74+01.32.
Methods.
We used data from the Wide-field Infrared Survey Explorer (WISE), Infrared Astronomical Satellite (IRAS), and
Herschel
in conjuction with our previous 350 and 870
μ
m observations with the Submillimetre APEX Bolometer Camera (SABOCA) and Large APEX BOlometer CAmera, and constructed the far-IR to submillimetre spectral energy distributions (SEDs) of the cores. The SEDs were fitted using single or two-temperature modified blackbody emission curves to derive the dust temperatures, masses, and luminosities of the cores.
Results.
For the 12 analysed cores, which include two IR dark cores (no WISE counterpart), nine IR bright cores, and one H
II
region, the mean dust temperature of the cold (warm) component, the mass, luminosity, H
2
number density, and surface density were derived to be 13.3 ± 1.4 K (47.0 ± 5.0 K), 113 ± 29
M
⊙
, 192 ± 94
L
⊙
, (4.3 ± 1.2) × 10
5
cm
−3
, and 0.77 ± 0.19 g cm
−3
, respectively. The H
II
region IRAS 13039-6108a was found to be the most luminous source in our sample ((1.1 ± 0.4) × 10
3
L
⊙
). All the cores were found to be gravitationally bound (i.e. the virial parameter
α
vir
< 2). Two out of the nine analysed IR bright cores (22%) were found to follow an accretion luminosity track under the assumptions that the mass accretion rate is 10
−5
M
⊙
yr
−1
, the stellar mass is 10% of the parent core mass, and the radius of the central star is 5
R
⊙
. Most of the remaing ten cores were found to lie within 1 dex below this accretion luminosity track. Seven out of 12 of the analysed cores (58%) were found to lie above the mass-radius thresholds of high-mass star formation proposed in the literature. The surface densities of Σ > 0.4 g cm
−3
derived for these seven cores also exceed the corresponding threshold for high-mass star formation. Five of the analysed cores (42%) show evidence of fragmentation into two components in the SABOCA 350
μ
m image.
Conclusions.
In addition to the H
II
region source IRAS 13039-6108a, some of the other cores in Seahorse also appear to be capable of giving birth to high-mass stars. The 22
μ
m dark core SMM 9 is likely to be the youngest source in our sample that has the potential to form a high-mass star (96 ± 23
M
⊙
within a radius of ~0.1 pc). The dense core population in the Seahorse IRDC has comparable average properties to the cores in the well-studied Snake IRDC G11.11-0.12 (e.g.
T
dust
and
L
agree within a factor of ~1.8); furthermore, the Seahorse, which lies ~60 pc above the Galactic plane, appears to be a smaller (e.g. three times shorter in projection, ~100 times less massive) version of the Snake. The Seahorse core fragmentation mechanisms appear to be heterogenous, including cases of both thermal and non-thermal Jeans instability. High-resolution follow-up studies are required to address the fragmented cores’ genuine potential of forming high-mass stars.
Context.
Physically unassociated background or foreground objects seen towards submillimetre sources are potential contaminants of both the studies of young stellar objects embedded in Galactic dust ...clumps and multiwavelength counterparts of submillimetre galaxies (SMGs).
Aims.
We aim to search for and characterise the properties of a potential extragalactic object seen in projection towards a Galactic dust clump.
Methods.
We employed the near-infrared (3.4
μ
m and 4.6
μ
m) and mid-infrared (12
μ
m and 22
μ
m) data from the Wide-field Infrared Survey Explorer (WISE) and the submillimetre data from the
Planck
satellite.
Results.
We uncovered a source, namely the WISE source J044232.92+322734.9 (hereafter J044232.92), which is detected in the W1–W3 bands of WISE, but undetected at 22
μ
m (W4), and whose WISE infrared (IR) colours suggest that it is a star-forming galaxy (SFG). This source is seen in projection towards the
Planck
-detected dust clump PGCC G169.20-8.96, which likely belongs to the Taurus-Auriga cloud complex, at a distance of 140 pc. We used the
MAGPHYS+photo-
z
spectral energy distribution (SED) code to derive the photometric redshift and physical properties of J044232.92. The redshift was derived to be
z
phot
= 1.132
−0.165
+0.280
, while, for example, the stellar mass, IR (8–1000
μ
m) luminosity, and star formation rate were derived to be
M
⋆
= 4.6
−2.5
+4.7
× 10
11
M
⊙
,
L
IR
= 2.8
−1.5
+5.7
× 10
12
L
⊙
, and SFR = 191
−146
+580
M
⊙
yr
−1
(or 281
−155
+569
M
⊙
yr
−1
when estimated from the IR luminosity). The derived value of
L
IR
suggests that J044232.92 could be an ultraluminous IR galaxy, and we found that it is consistent with a main sequence SFG at a redshift of 1.132.
Conclusions.
The estimated physical properties of J044232.92 are comparable to those of SMGs, except that the derived stellar mass of J044232.92 appears somewhat higher (by a factor of 4–5) than the average stellar masses of SMGs. However, the stellar mass difference could just reflect the poorly sampled SED in the ultraviolet, optical, and near-IR regimes. Indeed, the SED of J044232.92 could not be well constrained using the currently available data (WISE only), and hence the derived redshift of the source and its physical properties should be taken as preliminary estimates. Further observations, in particular high-resolution (sub-)millimetre and radio continuum imaging, are needed to better constrain the redshift and physical properties of J044232.92 and to see if the source really is a galaxy seen through a Galactic dust clump, in particular an SMG population member at
z
∼ 1.1.
Context. Initial conditions and very early stages of star formation can be probed through spectroscopic observations of deuterated molecular species Aims. We aim to determine the ortho-H2D+ ...properties (e.g. column density and fractional abundance with respect to H2) in a sample of dense cores in the Orion B9 star-forming filament, and to compare those with the previously determined source characteristics, in particular with the gas kinetic temperature, N2D+/N2H+ deuterium fractionation, and level of CO depletion. Methods. We used the Atacama Pathfinder EXperiment (APEX) telescope to observe the 372 GHz o-H2D+(JKa, Kc = 11, 0−11, 1) line towards three prestellar cores and three protostellar cores in Orion B9. We also employed our previous APEX observations of C17O, C18O, N2H+, and N2D+ line emission, and 870 μm dust continuum emission towards the target sources. Results. The o-H2D+(11, 0−11, 1) line was detected in all three prestellar cores, but in only one of the protostellar cores. The corresponding o-H2D+ abundances were derived to be ~ (12−30) × 10−11 and ~ 6 × 10−11. Two additional spectral lines, DCO+(5−4) and N2H+(4−3), were detected in the observed frequency bands with high detection rates of 100 and 83%, respectively. We did not find any significant correlations among the explored parameters, although our results are mostly consistent with theoretical expectations. Also, the Orion B9 cores were found to be consistent with the relationship between theo-H2D+ abundance and gas temperature obeyed by other low-mass dense cores. The o-H2D+ abundance was found to decrease as the core evolves. Conclusions. The o-H2D+ abundances in the Orion B9 cores are in line with those found in other low-mass dense cores and larger than derived for high-mass star-forming regions. The higher o-H2D+ abundance in prestellar cores compared to that in cores hosting protostars is to be expected from chemical reactions where higher concentrations of gas-phase CO and elevated gas temperature accelerate the destruction of H2D+. The validity of using the o-H2D+/N2D+ abundance ratio as an evolutionary indicator, which has been proposed for massive clumps, remains to be determined when applied to these target cores. Similarly, the behaviour of the o-H2D+/DCO+ ratio as the source evolves was found to be ambiguous. Still larger samples and observations of additional deuterated species are needed to explore these potential evolutionary indicators further. The low radial velocity of the line emission from one of the targeted prestellar cores, SMM 7 (~ 3.6 km s−1 versus the systemic Orion B9 velocity of ~ 9 km s−1), suggests that it is a chance superposition seen towards Orion B9. Overall, as located in a dynamic environment of the Orion B molecular cloud, the Orion B9 filament provides an interesting target system to investigate the deuterium-based chemistry, and further observations of species like para-H2D+ and D2H+ would be of particular interest.
Context. Filamentary molecular clouds, such as many of the infrared dark clouds (IRDCs), can undergo hierarchical fragmentation into substructures (clumps and cores) that can eventually collapse to ...form stars. Aims. We aim to determine the occurrence of fragmentation into cores in the clumps of the filamentary IRDC G304.74+01.32 (hereafter, G304.74). We also aim to determine the basic physical characteristics (e.g. mass, density, and young stellar object (YSO) content) of the clumps and cores in G304.74. Methods. We mapped the G304.74 filament at 350 μm using the Submillimetre APEX Bolometer Camera (SABOCA) bolometer. The new SABOCA data have a factor of 2.2 times higher resolution than our previous Large APEX BOlometer CAmera (LABOCA) 870 μm map of the cloud (9″ vs. \hbox{$19\farcs86$}19 .̋ 86). We also employed the Herschel far-infrared (IR) and submillimetre, and Wide-field Infrared Survey Explorer (WISE) IR imaging data available for G304.74. The WISE data allowed us to trace the IR emission of the YSOs associated with the cloud. Results. The SABOCA 350 μm data show that G304.74 is composed of a dense filamentary structure with a mean width of only 0.18 ± 0.05 pc. The percentage of LABOCA clumps that are found to be fragmented into SABOCA cores is 36% ± 16%, but the irregular morphology of some of the cores suggests that this multiplicity fraction could be higher. The WISE data suggest that 65% ± 18% of the SABOCA cores host YSOs. The mean dust temperature of the clumps, derived by comparing the Herschel 250, 350, and 500 μm flux densities, was found to be 15.0 ± 0.8 K. The mean mass, beam-averaged H2 column density, and H2 number density of the LABOCA clumps are estimated to be 55 ± 10M⊙, (2.0 ± 0.2) × 1022 cm-2, and (3.1 ± 0.2) × 104 cm-3. The corresponding values for the SABOCA cores are 29 ± 3M⊙, (2.9 ± 0.3) × 1022 cm-2, and (7.9 ± 1.2) × 104 cm-3. The G304.74 filament is estimated to be thermally supercritical by a factor of ≳ 3.5 on the scale probed by LABOCA, and by a factor of ≳ 1.5 for the SABOCA filament. Conclusions. Our data strongly suggest that the IRDC G304.74 has undergone hierarchical fragmentation. On the scale where the clumps have fragmented into cores, the process can be explained in terms of gravitational Jeans instability. Besides the filament being fragmented, the finding of embedded YSOs in G304.74 indicates its thermally supercritical state, although the potential non-thermal (turbulent) motions can render the cloud a virial equilibrium system on scale traced by LABOCA. The IRDC G304.74 has a seahorse-like morphology in the Herschel images, and the filament appears to be attached by elongated, perpendicular striations. This is potentially evidence that G304.74 is still accreting mass from the surrounding medium, and the accretion process can contribute to the dynamical evolution of the main filament. One of the clumps in G304.74, IRAS 13039-6108, is already known to be associated with high-mass star formation, but the remaining clumps and cores in this filament might preferentially form low and intermediate-mass stars owing to their mass reservoirs and sizes. Besides the presence of perpendicularly oriented, dusty striations and potential embedded intermediate-mass YSOs, G304.74 is a relatively nearby (d ~ 2.5 kpc) IRDC, which makes it a useful target for future star formation studies. Owing to its observed morphology, we propose that G304.74 could be nicknamed the Seahorse Nebula.
Classification of young stellar objects (YSOs) into different evolutionary stages helps us to understand the formation process of new stars and planetary systems. Such classification has ...traditionally been based on spectral energy distribution (SED) analysis. An alternative approach is provided by supervised machine learning algorithms, which can be trained to classify large samples of YSOs much faster than via SED analysis. We attempt to classify a sample of Orion YSOs (the parent sample size is 330) into different classes, where each source has already been classified using multiwavelength SED analysis. We used eight different learning algorithms to classify the target YSOs, namely a decision tree, random forest, gradient boosting machine (GBM), logistic regression, naïve Bayes classifier,
k
-nearest neighbour classifier, support vector machine, and neural network. The classifiers were trained and tested by using a 10-fold cross-validation procedure. As the learning features, we employed ten different continuum flux densities spanning from the near-infrared to submillimetre wavebands (
λ
=
3.6
–
870
μ
m
). With a classification accuracy of 82% (with respect to the SED-based classes), a GBM algorithm was found to exhibit the best performance. The lowest accuracy of 47% was obtained with a naïve Bayes classifier. Our analysis suggests that the inclusion of the
3.6
μ
m
and
24
μ
m
flux densities is useful to maximise the YSO classification accuracy. Although machine learning has the potential to provide a rapid and fairly reliable way to classify YSOs, an SED analysis is still needed to derive the physical properties of the sources (e.g. dust temperature and mass), and to create the labelled training data. The machine learning classification accuracies can be improved with respect to the present results by using larger data sets, more detailed missing value imputation, and advanced ensemble methods (e.g. extreme gradient boosting). Overall, the application of machine learning is expected to be very useful in the era of big astronomical data, for example to quickly assemble interesting target source samples for follow-up studies.
Context. Infrared dark clouds (IRDCs) are promising sites to study the earliest formation stages of stellar clusters and high-mass stars, and the physics of molecular-cloud formation and ...fragmentation. Aims. We attempt to improve our understanding of the physical and chemical properties of the filamentary IRDC G304.74+01.32 (hereafter, G304.74). In particular, we investigate the kinematical and dynamical state of the cloud and clumps within it, and the amount of CO depletion. Methods. All of the submillimetre peak positions in the cloud identified from our previous LABOCA 870-μm map were observed in C17O(2−1) with APEX. These are the first line observations along the whole filament that have been made so far. Selected positions were also observed in the 13CO(2−1), SiO(5−4), and CH3OH(5k − 4k) transitions at ~1 mm. Results. The C17O lines were detected towards all target positions at similar radial velocities. CO does not appear to be significantly depleted in the clumps, the largest depletion factors being only about 2. Two to three methanol 5k − 4k lines near ~241.8 GHz were detected towards all selected positions, whereas SiO(5−4) was seen in only one of these positions, namely SMM 3. In the band covering SiO(5−4), we also detected the DCN(3−2) line towards SMM 3. The 13CO(2−1) lines display blue asymmetric profiles, which are indicative of large-scale infall motions. The clumps show transonic to supersonic non-thermal motions, and a virial-parameter analysis suggests that most of them are gravitationally bound. The external pressure may also play a non-negligible role in the dynamics. Our analysis suggests that the fragmentation of the filament into clumps is caused by a “sausage”-type instability, in agreement with results from other IRDCs. Conclusions. The uniform C17O radial velocities along the G304.74 cloud shows that it is a coherent filamentary structure. Although the clumps appear to be gravitationally bound, the ambient turbulent ram pressure may be an important factor in the cloud dynamics. This is qualitatively consistent with our earlier suggestion that the filament was formed by converging supersonic turbulent flows. The poloidal magnetic field could resist the radial cloud collapse, which conforms to the low infall velocites that we derived. The cloud may be unable to form high-mass stars based on the mass-size threshold. The star-formation activity in the cloud, such as outflows, is likely responsible for the release of CO from the icy grain mantles back into the gas phase. Shocks related to outflows may also have injected CH3OH, SiO, and DCN into the gas-phase in SMM 3.
We present the VLA-COSMOS 3 GHz Large Project based on 384 h of observations with the Karl G. Jansky Very Large Array (VLA) at 3 GHz (10 cm) toward the two square degree Cosmic Evolution Survey ...(COSMOS) field. The final mosaic reaches a median rms of 2.3 μJy beam-1 over the two square degrees at an angular resolution of 0.75″. To fully account for the spectral shape and resolution variations across the broad (2 GHz) band, we image all data with a multiscale, multifrequency synthesis algorithm. We present a catalog of 10 830 radio sources down to 5σ, out of which 67 are combined from multiple components.Comparing the positions of our 3 GHz sources with those from the Very Long Baseline Array (VLBA)-COSMOS survey, we estimate that the astrometry is accurate to 0.01″ at the bright end (signal-to-noise ratio, S/N3 GHz > 20). Survival analysis on our data combined with the VLA-COSMOS 1.4 GHz Joint Project catalog yields an expected median radio spectral index of α = −0.7. We compute completeness corrections via Monte Carlo simulations to derive the corrected 3 GHz source counts. Our counts are in agreement with previously derived 3 GHz counts based on single-pointing (0.087 square degrees) VLA data. In summary, the VLA-COSMOS 3 GHz Large Project simultaneously provides the largest and deepest radio continuum survey at high (0.75″) angular resolution to date, bridging the gap between last-generation and next-generation surveys.
Context. Imaging surveys of dust emission at (sub)millimetre wavelengths provide a powerful tool for studying molecular clouds and the early stages of star formation. Aims. Through submm dust ...continuum mapping, we attempt to search for genuine infrared-dark clouds (IRDCs) and precursors to massive stars and stellar clusters in the Galactic plane, and to determine their basic physical properties. Methods. We have mapped four selected fields of about 0.°5×0.°5 that contain Spitzer 8-μm dark regions with LABOCA at 870 μm. Selected positions in the fields were observed in C17O(2−1) to obtain kinematic information. The obtained LABOCA maps are used in conjunction with the Spitzer IR images. Results. The total number of clumps identified in this survey is 91, out of which 40 (44%) appear dark at 8 and 24 μm. The remaining clumps are associated with mid-IR emission. Seven clumps associated with extended 4.5 μm emission are candidate extended green objects (EGOs). Filamentary dust “ridges” were found towards the Spitzer bubbles N10/11 in one of our fields. The relative number of IR-dark and IR-bright clumps suggests that the duration of the former stage is about 1.6 × 105 yr. The mass distribution of the total sample of clumps, and that separately constructed for the IR-dark and IR-bright clumps, could be fitted at the high-mass end with the power-law function dN/dlog M ∝ M−Γ, where Γ ≃ 0.7...0.8. The C17O observation positions appear to be dominated by non-thermal motions, and the data also revealed some potential sites of strong CO depletion. In G11.36+0.80, which is the best example of a filamentary IRDC in our sample, the clumps appear to be gravitationally bound. The fragmentation of the filament can be understood in terms of a sausage-type fluid instability, in agreement with the results for other IRDCs. The fragmentation and the CO depletion timescales in G11.36 appear to be very similar to each other. Conclusions. Many of the identified clumps are massive enough to allow high-mass star formation, and some of them already show clear signposts of that. In the N10/11 bubble environment, the morphology of the detected dust emission conforms to the triggered high-mass star formation in the system. The clump mass distributions are similar to those found for diffuse CO clumps, and can be explained by the action of supersonic turbulence. The formation of filamentary IRDCs might be caused by converging turbulent flows, and the same process may play a role in exciting the fluid perturbations responsible for the fragmentation of the clouds into clumps.
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