The Dusty Galactic Center as Seen by SCUBA-2 Parsons, H.; Dempsey, J. T.; Thomas, H. S. ...
The Astrophysical journal. Supplement series,
02/2018, Letnik:
234, Številka:
2
Journal Article
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We present new JCMT SCUBA-2 observations of the Galactic Center region from and , covering 10 × 2 square degrees along the Galactic Plane to a depth of 43 mJy beam−1 at 850 m and 360 mJy beam−1 at ...450 m. We describe the mapping strategy and reduction method used. We present 12CO(3-2) observations of selected regions in the field. We derive the molecular-line conversion factors (mJy beam−1 per K km s−1) at 850 and 450 m, which are then used to obtain the amount of contamination in the continuum maps due to 12CO(3-2) emission in the 850 m band. Toward the fields where the CO contamination has been accounted for, we present an 850 m CO-corrected compact source catalog. Finally, we look for possible physical trends in the CO contamination with respect to column density, mass, and concentration. No trends were seen in the data despite the recognition of three contributors to CO contamination: opacity, shocks, and temperature, which would be expected to relate to physical conditions. These SCUBA-2 Galactic Center data and catalog are available via https://doi.org/10.11570/17.0009.
Context.
NGC 6357 is a galactic star-forming complex (
d
~ 1.7 kpc) composed of several H
II
regions, a few young stellar clusters, and giant molecular clouds. In particular, the H
II
regions ...G353.2+0.9, G353.1+0.6, and G353.2+0.7 are associated with three young clusters; the most prominent of these, Pismis 24, contains some of the most massive stars known.
Aims.
We aim to derive the properties of the densest compact gas structures (cores) in the region as well as the effects of an intense far-ultraviolet (FUV) radiation field on their global properties.
Methods.
We mapped the NGC 6357 region at 450 and 850 μm with SCUBA-2 and in the CO(3–2) line with HARP at the JCMT. We also made use of the
Herschel
Hi-GAL data at 70 and 160 μm. We used the algorithm Gaussclumps to retrieve the compact cores embedded in the diffuse sub-millimetre emission and constructed their spectral energy distribution from 70 to 850 μm, from which we derived mass and temperature. We divided the observed area into an ‘active’ region (i.e. the eastern half, which is exposed to the FUV radiation from the more massive members of the three clusters) and a ‘quiescent’ region (i.e. the western half, which is less affected by FUV radiation). We compared the core mass functions and the temperature distributions in the two areas to look for any differences that could be due to the different levels of FUV radiation.
Results.
We retrieved 686 dense cores, 411 in the active region and 275 in the quiescent region, with an estimated mass completeness limit of ~5
M
⊙
. We also attempted to select a sample of pre-stellar cores based on cross-correlation with 70 μm emission and red WISE point sources, which unfortunately is biased due to distance, emission at 70 μm from the dust on the surface of the cores that is heated by the FUV radiation, and saturation in the WISE bands. Most of the cores above the mass completeness limit are likely to be gravitationally bound. The fraction of gas in dense cores is very low, 1.4%. We found a mass-size relation log(
M
∕
M
⊙
) ~
a
× log(
D
∕arcsec), with
a
in the range 2.0–2.4, depending on the precise selection of the sample. The temperature distributions in the two sub-regions are clearly different, peaking at ~25 K in the quiescent region and at ~35 K in the active region. The core mass functions are different as well, at a 2
σ
level, consistent with a Salpeter initial mass function in the quiescent region and flatter than that in the active region. The dense cores lying close to the H
II
regions are consistent with pre-existing cores being gradually engulfed by a photon dominated region and photoevaporating. A comparison of the obtained distribution of core masses with those derived from simulations of cloud-cloud collisions yields no conclusive evidence of ongoing cloud-cloud collisions.
Conclusions.
We attribute the different global properties of dense cores in the two sub-regions to the influence of the FUV radiation field.
The Gould Belt Legacy Survey will survey nearby star-forming regions (within 500 pc), using Heterodyne Array Receiver Programme (HARP), Submillimetre Common-User Bolometer Array 2 and Polarimeter 2 ...on the James Clerk Maxwell Telescope. This paper describes the initial data obtained using HARP to observe 12CO, 13CO and C18O J= 3 → 2 towards two regions in Orion B, NGC 2024 and NGC 2071. We describe the physical characteristics of the two clouds, calculating temperatures and opacities utilizing all the three isotopologues. We find good agreement between temperatures calculated from CO and from dust emission in the dense, energetic regions. We determine the mass and energetics of the clouds, and of the high-velocity material seen in 12CO emission, and compare the relative energetics of the high- and low-velocity material in the two clouds. We present a clumpfind analysis of the 13CO condensations. The slope of the condensation mass functions, at the high-mass ends, is similar to the slope of the initial mass function.
Context. This paper relates to low-mass star formation in globules, and the interaction of newly-formed stars with their environment. We follow up on the results of our earlier observations of this ...globule. Aims. Our aim is to study the gas- and dust environment of the young stellar object (YSO) in globule CB88 230, the large-scale molecular outflow triggered by the jet driven by the YSO, and their interaction. Methods. We carried out submillimetre continuum and multi-line molecular observations with several single-dish facilities, mapping the core of the globule and the large-scale outflow associated with the YSO. Results. Dust continuum and molecular line maps (of 12CO, C18O, CS, CH3OH) show a flattened (axes ratio 1.5−1.7), asymmetric core with a full width at half maximum (FWHM)-diameter of 0.16−0.21 pc. Line profiles of 12CO, 13CO(2–1, 3–2), and CS(2–1) show self-absorption near the YSO; the absorption dip is at a slightly (~0.3 km s−1) redder velocity than that of the quiescent gas, possibly indicating infall of cooler envelope gas. The mass of the core, determined from C18O(1–0) observations, is about 8 M⊙, while the virial mass is in the range 5−8M⊙, depending on the assumed density distribution. We detect a slight velocity gradient (~0.98 km s−1 pc−1), though rotational energy is negligible with respect to gravitational and turbulent energy of the core. A fit to the spectral energy distribution of the core gives a dust temperature Td ≈ 18 K and a gas mass of ca. 2 M⊙ (assuming a gas-to-dust ratio of 100). More careful modelling of the sub-mm emission (not dominated by the relatively hot central regions) yields M ≈ 8M⊙. From the molecular line observations we derive gas temperatures of 10−20 K. A Bayesian analysis of the emission of selected molecules observed towards the YSO, yields Tkin ≈ 21.4 K (68% credibility interval 14.5−35.5 K) and volume density n(H2) ≈ 4.6 × 105 cm−3 (8.3 × 104−9.1 × 105 cm−3). We have mapped the well-collimated large-scale outflow in 12CO(3–2). The outflow has a dynamical age of a few 104 yr, and contains little mass (a few 10−2 M⊙). A misalignment between the axis of this large-scale outflow and that of the hot jet close to the YSO indicates that the outflow direction may be changing with time.
As part of a James Clerk Maxwell Telescope (JCMT) Legacy Survey of star formation in the Gould Belt, we present early science results for Taurus. CO J= 3 –2 maps have been secured along the ...north-west ridge and bowl, collectively known as L 1495, along with deep 13CO and C18O J= 3 –2 maps in two subregions. With these data, we search for molecular outflows, and use the distribution of flows, Herbig–Haro (HH) objects and shocked H2 line-emission features, together with the population of young stars, protostellar cores and starless condensations to map star formation across this extensive region. In total, 21 outflows are identified. It is clear that the bowl is more evolved than the ridge, harbouring a greater population of T Tauri stars and a more diffuse, more turbulent ambient medium. By comparison, the ridge contains a much younger, less widely distributed population of protostars which, in turn, is associated with a greater number of molecular outflows. We estimate the ratio of the numbers of pre-stellar to protostellar cores in L 1495 to be ∼1.3–2.3, and of gravitationally unbound starless cores to (gravitationally bound) pre-stellar cores to be ∼1. If we take previous estimates of the protostellar lifetime of ∼5 × 105 yr, this indicates a pre-stellar lifetime of 9(±3) × 105 yr. From the number of outflows, we also crudely estimate the star formation efficiency in L 1495, finding it to be compatible with a canonical value of 10–15 per cent. We note that molecular outflow-driving sources have redder near-infrared colours than their HH jet-driving counterparts. We also find that the smaller, denser cores are associated with the more massive outflows, as one might expect if mass build-up in the flow increases with the collapse and contraction of the protostellar envelope.
The sub-mm variability of IRC+10216 and o Ceti Dharmawardena, Thavisha E; Kemper, Francisca; Wouterloot, Jan G A ...
Monthly notices of the Royal Astronomical Society,
11/2019, Letnik:
489, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Abstract
We present the sub-mm variability of two of the most well-studied AGB stars, IRC + 10216 and o Ceti. The data are obtained at $450$ and $850\,{\mu {\rm m}}$ as part of pointing calibration ...observations for the James Clerk Maxwell Telescope’s SCUBA-2 instrument over a span of 7 yr. The periods are derived using non-parametric methods, Gatspy Supersmoother and P4J, in order not to assume an underlying shape to the periodicity. These were compared to two Lomb–Scargle parametric methods. We find that for both sources and wavelengths the periods derived from all methods are consistent within 1σ. The $850\,{\mu {\rm m}}$ phase folded light curves of IRC + 10216 show a time lag of ∼540 d compared to its optical counterpart. We explore the origins of the sub-mm variability and the phase lag using radiative transfer models. Combining the modelling with findings in the literature, we find that the sub-mm emission and phase lag can be partially attributed to the dust formation or destruction cycle. A second, unknown mechanism must be invoked; we defer an investigation of the origin and nature of this mechanism to a future work.
ABSTRACT
We present the highest resolution single-dish submillimetre observations of the detached shell source U Antliae to date. The observations were obtained at $450$ and $850\,{\mu}{\rm m}$ with ...SCUBA-2 instrument on the James Clerk Maxwell Telescope as part of the Nearby Evolved Stars Survey. The emission at $850\,{\mu}{\rm m}$ peaks at 40 arcsec with hints of a second peak seen at ∼20 arcsec. The emission can be traced out to a radius of 56 arcsec at a 3σ level. The outer peak observed at $850\,{\mu}{\rm m}$ aligns well with the peak observed at Herschel/PACS wavelengths. With the help of spectral energy distribution fitting and radiative transfer calculations of multiple-shell models for the circumstellar envelope, we explore the various shell structures and the variation of grain sizes along the in the circumstellar envelope. We determine a total shell dust mass of (2.0 ± 0.3) × 10−5 M⊙ and established that the thermal pulse that gave rise to the detached shell occurred 3500 ± 500 yr ago.
CH abundance gradient in TMC-1 Suutarinen, A.; Geppert, W. D.; Harju, J. ...
Astronomy and astrophysics (Berlin),
07/2011, Letnik:
531
Journal Article
Recenzirano
Odprti dostop
Aims. The aim of this study is to examine if the well-known chemical gradient in TMC-1 is reflected in the amount of rudimentary forms of carbon available in the gas-phase. As a tracer we use the CH ...radical which is supposed to be well correlated with carbon atoms and simple hydrocarbon ions. Methods. We observed the 9-cm Λ-doubling lines of CH along the dense filament of TMC-1. The CH column densities were compared with the total H2 column densities derived using the 2MASS NIR data and previously published SCUBA maps and with OH column densities derived using previous observations with Effelsberg. We also modelled the chemical evolution of TMC-1 adopting physical conditions typical of dark clouds using the UMIST Database for Astrochemistry gas-phase reaction network to aid the interpretation of the observed OH/CH abundance ratios. Results. The CH column density has a clear peak in the vicinity of the cyanopolyyne maximum of TMC-1. The fractional CH abundance relative to H2 increases steadily from the northwestern end of the filament where it lies around 1.0 × 10-8, to the southeast where it reaches a value of 2.0 × 10-8. The OH and CH column densities are well correlated, and we obtained OH/CH abundance ratios of ~16–20. These values are clearly larger than what has been measured recently in diffuse interstellar gas and is likely to be related to C to CO conversion at higher densities. The good correlation between CH and OH can be explained by similar production and destruction pathways. We suggest that the observed CH and OH abundance gradients are mainly due to enhanced abundances in a low-density envelope which becomes more prominent in the southeastern part and seems to continue beyond the dense filament. Conclusions. An extensive envelope probably signifies an early stage of dynamical evolution, and conforms with the detection of a large CH abundance in the southeastern part of the cloud. The implied presence of other simple forms of carbon in the gas phase provides a natural explanation for the observation of “early-type” molecules in this region.
We present $\ion{C}{i}$ 3P1–3P0 spectra at four spiral arm positions and the nuclei of the nearby galaxies M 83 and M 51 obtained at the JCMT. The spiral arm positions lie at galacto-centric ...distances of between 2 kpc and 6 kpc. This data is complemented with maps of CO 1–0, 2–1, and 3–2, and ISO/LWS far-infrared data of $\ion{C}{ii}$ (158 μm), $\ion{O}{i}$ (63 μm), and $\ion{N}{ii}$ (122 μm) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the $\ion{N}{ii}$ line, we estimate that between 15% and 30% of the observed $\ion{C}{ii}$ emission originates from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below ~$0.21\%$ in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) with the standard ratios $\ion{O}{i}$(63)/$\ion{C}{ii}$PDR and ($\ion{O}{i}$(63)+$\ion{C}{ii}$PDR) vs. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, $\ion{C}{ii}$$_{\rm PDR}$/$\ion{C}{i}$(1–0), $\ion{C}{i}$(1–0)/CO(3–2), CO(3–2)/CO(1–0), $\ion{C}{ii}$/CO(3–2), and, $\ion{O}{i}$(63)/$\ion{C}{ii}$PDR. The best fits to the latter ratios yield densities of 104 cm-3 and FUV fields of ~$G_0=20$–30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4–5 at the nuclei, indicating the presence of other mechanisms heating the dust. The $\ion{C}{i}$ area filling factors lie below 2% at all positions, consistent with low volume filling factors of the emitting gas. The fit of the model to the line ratios improves significantly if we assume that $\ion{C}{i}$ stems from a larger region than CO 2–1. Improved modelling would need to address the filling factors of the various submm and FIR tracers, taking into consideration the presence of density gradients of the emitting gas by including cloud mass and size distributions within the beam.
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