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Register a new userA multiwavelength study of young massive star forming regions: II. The dust environment
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We present observations of 1.2-mm dust continuum emission, made with the Swedish ESO Submillimeter Telescope, towards eighteen luminous IRAS point sources, all with colors typical of compact HII regions and associated with CS(2-1) emission, thought to be representative of young massive star forming regions. Emission was detected toward all the IRAS objects. We find that the 1.2-mm sources associated with them have distinct physical parameters, namely sizes of 0.4 pc, dust temperatures of 30 K, masses of 2x10^3 Msun, column densities of 3x10^23 cm^-2, and densities of 4x10^5 cm^-3. We refer to these dust structures as massive and dense cores. Most of the 1.2-mm sources show single-peaked structures, several of which exhibit a bright compact peak surrounded by a weaker extended envelope. The observed radial intensity profiles of sources with this type of morphology are well fitted with power-law intensity profiles with power-law indices in the range 1.0-1.7. This result indicates that massive and dense cores are centrally condensed, having radial density profiles with power-law indices in the range 1.5-2.2. We also find that the UC HII regions detected with ATCA towards the IRAS sources investigated here (Paper I) are usually projected at the peak position of the 1.2-mm dust continuum emission, suggesting that massive stars are formed at the center of the centrally condensed massive and dense cores.
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We present mid-infrared (MIR) observations, made with the TIMMI2 camera on the ESO 3.6 m telescope, toward 14 young massive star-forming regions. All regions were imaged in the N band, and nine in the Q band, with an angular resolution of ~ 1 arcsec. Typically, the regions exhibit a single or two compact sources (with sizes in the range 0.008-0.18 pc) plus extended diffuse emission. The Spitzer-Galactic Legacy Infrared Mid-Plane Survey Extraordinaire images of these regions show much more extended emission than that seen by TIMMI2, and this is attributed to polycyclic aromatic hydrocarbon (PAH) bands. For the MIR sources associated with radio continuum radiation (Paper I) there is a close morphological correspondence between the two emissions, suggesting that the ionized gas (radio source) and hot dust (MIR source) coexist inside the H II region. We found five MIR compact sources which are not associated with radio continuum emission, and are thus prime candidates for hosting young massive protostars. In particular, objects IRAS 14593-5852 II (only detected at 17.7 microns) and 17008-4040 I are likely to be genuine O-type protostellar objects. We also present TIMMI2 N-band spectra of eight sources, all of which are dominated by a prominent silicate absorption feature (~ 9.7 microns). From these data we estimate column densities in the range (7-17)x10^22 cm^-2, in good agreement with those derived from the 1.2 mm data (Paper II). Seven sources show bright [Ne II] line emission, as expected from ionized gas regions. Only IRAS 123830-6128 shows detectable PAH emission at 8.6 and 11.3 microns.
We present a multiwavelength study of 28 Galactic massive star-forming H II regions. For 17 of these regions, we present new distance measurements based on Gaia DR2 parallaxes. By fitting a multicomponent dust, blackbody, and power-law continuum model to the 3.6 $mu$m through 10 mm spectral energy distributions, we find that ${sim}34$% of Lyman continuum photons emitted by massive stars are absorbed by dust before contributing to the ionization of H II regions, while ${sim}68$% of the stellar bolometric luminosity is absorbed and reprocessed by dust in the H II regions and surrounding photodissociation regions. The most luminous, infrared-bright regions that fully sample the upper stellar initial mass function (ionizing photon rates $N_C ge 10^{50}~{rm s}^{-1}$ and dust-processed $L_{rm TIR}ge 10^{6.8}$ L$_{odot}$) have on average higher percentages of absorbed Lyman continuum photons ($sim$51%) and reprocessed starlight ($sim$82%) compared to less luminous regions. Luminous H II regions show lower average PAH fractions than less luminous regions, implying that the strong radiation fields from early-type massive stars are efficient at destroying PAH molecules. On average, the monochromatic luminosities at 8, 24, and 70 $mu$m combined carry 94% of the dust-reprocessed $L_{rm TIR}$. $L_{70}$ captures ${sim}52$% of $L_{rm TIR}$, and is therefore the preferred choice to infer the bolometric luminosity of dusty star-forming regions. We calibrate SFRs based on $L_{24}$ and $L_{70}$ against the Lyman continuum photon rates of the massive stars in each region. Standard extragalactic calibrations of monochromatic SFRs based on population synthesis models are generally consistent with our values.
We report trigonometric parallaxes for the sources NGC 7538 and Cep A, corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04] kpc, respectively. The distance to NGC 7538 is considerably smaller than its kinematic distance and places it in the Perseus spiral arm. The distance to Cep A is also smaller than its kinematic distance and places it in the Local arm or spur. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find significant deviations from circular Galactic orbits for these sources: both sources show large peculiar motions (> 10 km/s) counter to Galactic rotation and NGC 7538 has a comparable peculiar motion toward the Galactic center.
We present a multiwavelength study of the massive star forming region associated with IRAS 06055+2039 which reveals an interesting scenario of this complex where regions are at different stages of evolution of star formation. Narrow band near-infrared (NIR) observations were carried out with UKIRT-UFTI in molecular hydrogen and Br$gamma$ lines to trace the shocked and ionized gases respectively. We have used 2MASS $J H K_{s}$ data to study the nature of the embedded cluster associated with IRAS 06055+2039. We obtain a power-law slope of 0.43$pm$0.09 for the $K_{s}$-band Luminosity Function (KLF) which is in good agreement with other young embedded clusters. We estimate an age of 2 -- 3 Myr for this cluster. The radio emission from the ionized gas has been mapped at 610 and 1280 MHz using the Giant Metrewave Radio Telescope (GMRT), India. Apart from the diffuse emission, the high resolution 1280 MHz map also shows the presence of several discrete sources which possibly represent high density clumps. The morphology of shocked molecular hydrogen forms an arc towards the N-E of the central IRAS point source and envelopes the radio emission. Submillimetre emission using JCMT-SCUBA show the presence of a dense cloud core which is probably at an earlier evolutionary stage compared to the ionized region with shocked molecular gas lying in between the two. Emission from warm dust and the Unidentified Infrared Bands (UIBs) have been estimated using the mid-infrared (8 -- 21 $mu$m) data from the MSX survey. From the submillimetre emission at 450 and 850 $mu$m the total mass of the cloud is estimated to be $sim$ 7000 -- 9000 $rm M_{odot}$.
Imaging polarimetry is a useful tool to reveal the 3D structure of dust distributions and to localize embedded young stellar objects. We present maps of the linear polarization at 2.2 micron for three ultra-compact HII regions (G192.16-3.82, G331.28-0.19, G339.88-1.26) and the methanol maser source G305.21+0.21. From the polarization maps, we draw conclusions on the morphology of these objects and the presence of luminous illuminating sources.
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