Bipolar H II regions Samal, M. R.; Deharveng, L.; Zavagno, A. ...
Astronomy and astrophysics (Berlin),
09/2018, Volume:
617
Journal Article
Peer reviewed
Open access
Aims. We aim to identify bipolar Galactic H II regions and to understand their parental cloud structures, morphologies, evolution, and impact on the formation of new generations of stars. Methods. We ...use the Spitzer-GLIMPSE, Spitzer-MIPSGAL, and Herschel-Hi-GAL surveys to identify bipolar H II regions and to examine their morphologies. We search for their exciting star(s) using NIR data from the 2MASS, UKIDSS, and VISTA surveys. Massive molecular clumps are detected near these bipolar nebulae, and we estimate their temperatures, column densities, masses, and densities. We locate Class 0/I young stellar objects (YSOs) in their vicinities using the Spitzer and Herschel-PACS emission. Results. Numerical simulations suggest bipolar H II regions form and evolve in a two-dimensional flat- or sheet-like molecular cloud. We identified 16 bipolar nebulae in a zone of the Galactic plane between ℓ ± 60° and |b| < 1°. This small number, when compared with the 1377 bubble H II regions in the same area, suggests that most H II regions form and evolve in a three-dimensional medium. We present the catalogue of the 16 bipolar nebulae and a detailed investigation for six of these. Our results suggest that these regions formed in dense and flat structures that contain filaments. We find that bipolar H II regions have massive clumps in their surroundings. The most compact and massive clumps are always located at the waist of the bipolar nebula, adjacent to the ionised gas. These massive clumps are dense, with a mean density in the range of 105 cm−3 to several 106 cm−3 in their centres. Luminous Class 0/I sources of several thousand solar luminosities, many of which have associated maser emission, are embedded inside these clumps. We suggest that most, if not all, massive 0/I YSO formation has probably been triggered by the expansion of the central bipolar nebula, but the processes involved are still unknown. Modelling of such nebula is needed to understand the star formation processes at play.
W5-E has been observed with the Herschel-PACS and -SPIRE photometers, at 100, 160, 250, 350, and 500 microns. The dust temperature map shows a rather uniform temperature, in the range 17.5-20 K in ...the dense condensations or filaments, 21-22 K in the photodissociation regions, and 24-31 K in the direction of the ionized regions. The column densities are rather low, everywhere lower than 10^23 cm-2, and of the order of a few 10^21 cm-2 in the PDRs. About 8000 solar masses of neutral material surrounds the ionized region, which is low with respect to the volume of this HII region; we suggest that the exciting stars of the W5-E, W5-W, Sh~201, A and B HII regions formed along a dense filament or sheet rather than inside a more spherical cloud. Fifty point sources have been detected at 100 microns. Most of them are Class 0/I YSOs. The SEDs of their envelopes have been fitted using a modified blackbody model. These envelopes are cold, with a mean temperature of 15.7+-1.8K. Their masses are in the range 1.3-47 solar masses. Eleven of these point sources are candidate Class 0 YSOs. Twelve of these point sources are possibly at the origin of bipolar outflows detected in this region. None of the YSOs contain a massive central object, but a few may form a massive star as they have both a massive envelope and also a high envelope accretion rate. Most of the Class 0/I YSOs are observed in the direction of high column density material, for example in the direction of the massive condensations present at the waist of the bipolar Sh 201 HII region or enclosed by the bright-rimmed cloud BRC14. The overdensity of Class 0/I YSOs on the borders of the HII regions strongly suggests that triggered star formation is at work in this region but, due to insufficient resolution, the exact processes at the origin of the triggering are difficult to determine.
Context. Star-forming complexes are large structures exhibiting massive star-formation at different stages of evolution, from dense cores to well-developed H ii regions. They are very interesting for ...the study of the formation and evolution of stars. NGC 6334 and NGC 6357 are two active and relatively nearby star-forming complexes. From the extinction map and the sub-mm cold dust emission, and because they have similar velocities, these regions are most likely connected. However, located in the direction of the Galactic center their radial velocity is not representative of their distance. An alternative is then to determine the distance of NGC 6334 and NGC 6357 from their stellar content. Aims. Our aim is to perform a census of O-B3 ionising stars in NGC 6334 and NGC 6357, to determine the extinction coefficient, and the distance of both regions. A census of O-B3 stars is an essential basis for estimating the statistical lifetime of the earliest massive star-forming phases. Methods. We performed a U, B, V, and R photometric survey of a large area covering NGC 6334 and NGC 6357 with the VIMOS (ESO-VLT) and the MOSAIC (CTIO) instruments. This allows us to have a complete census of O to B3 stars up to V = 22.6 mag. The OB stars are selected based on their U − B and B − V colors. The most robust extinction coefficient is determined from color − color plots before computing the distance of the OB stars. Results. We find a higher value than typical of the diffuse interstellar medium for RV of 3.53 ± 0.08 and 3.56 ± 0.15 for NGC 6357 and NGC 6334, respectively. Adopting these RV values, the distances of NGC 6357 and NGC 6334 are 1.9 ± 0.4 kpc and 1.7 ± 0.3 kpc. We conclude that, within the error bars, both regions are thus at the same distance of 1.75 kpc (weighted mean). We confirm that the value of RV is linked to the large dust grain content. In particular, we found that there are more very small grains in NGC 6357 than in NGC 6334, suggesting that NGC 6357 could be more evolved than NGC 6334. Placed in the Galactic context, the NGC 6334-NGC 6357 complex appears to be located at the inner edge of the Sagittarius-Carina arm. Our census of O to B3 stars leads to a count of ~230, which allows us to determine the statistical lifetime of the earliest phases of the massive stars. The starless and the protostellar phases have a mean statistical lifetime of ~1.5 × 104 yr and ~2.2 × 105 yr, respectively.
The origin and possible universality of the stellar initial mass function (IMF) is a major issue in astrophysics. One of the main objectives of the Herschel Gould Belt Survey is to clarify the link ...between the prestellar core mass function (CMF) and the IMF. We present and discuss the core mass function derived from Herschel data for the large population of prestellar cores discovered with SPIRE and PACS in the Aquila rift cloud complex at d ~ 260 pc. We detect a total of 541 starless cores in the entire ~11 deg2 area of the field imaged at 70–500 μm with SPIRE/PACS. Most of these cores appear to be gravitationally bound, and thus prestellar in nature. Our Herschel results confirm that the shape of the prestellar CMF resembles the stellar IMF, with much higher quality statistics than earlier submillimeter continuum ground-based surveys.
We present the initial highlights of the HOBYS key program, which are based on Herschel images of the Rosette molecular complex and maps of the RCW120 H ii region. Using both SPIRE at 250/350/500 μm ...and PACS at 70/160 μm or 100/160 μm, the HOBYS survey provides an unbiased and complete census of intermediate- to high-mass young stellar objects, some of which are not detected by Spitzer. Key core properties, such as bolometric luminosity and mass (as derived from spectral energy distributions), are used to constrain their evolutionary stages. We identify a handful of high-mass prestellar cores and show that their lifetimes could be shorter in the Rosette molecular complex than in nearby low-mass star-forming regions. We also quantify the impact of expanding H ii regions on the star formation process acting in both Rosette and RCW 120.
Context. Because of their relatively simple morphology, “bubble” H II regions have been instrumental to our understanding of star formation triggered by H II regions. With the far-infrared (FIR) ...spectral coverage of the Herschel satellite, we can access the wavelengths where these regions emit the majority of their energy through their dust emission. Aims. We wish to learn about the dust temperature distribution in and surrounding bubble H II regions and to calculate the mass and column density of regions of interest, in order to better understand ongoing star formation. Additionally, we wish to determine whether and how the spectral index of the dust opacity, β, varies with dust temperature. Any such relationship would imply that dust properties vary with environment. Methods. Using aperture photometry and fits to the spectral energy distribution, we determine the average temperature, β-value, and mass for regions of interest within eight bubble H II regions. Additionally, we compute maps of the dust temperature and column density. Results. At Herschel wavelengths (70 μm to 500 μm), the emission associated with H II regions is dominated by the cool dust in their photodissociation regions (PDRs). We find average dust temperatures of 26 K along the PDRs, with little variation between the H II regions in the sample, while local filaments and infrared dark clouds average 19 K and 15 K respectively. Higher temperatures lead to higher values of the Jeans mass, which may affect future star formation. The mass of the material in the PDR, collected through the expansion of the H II region, is between ~300 M⊙ and ~ 10 000 M⊙ for the H II regions studied here. These masses are in rough agreement with the expected masses swept up during the expansion of the H II regions. Approximately 20% of the total FIR emission is from the direction of the bubble central regions. This suggests that we are detecting emission from the “near-side” and “far-side” PDRs along the line of sight and that bubbles are three-dimensional structures. We find only weak support for a relationship between dust temperature and β, of a form similar to that caused by noise and calibration uncertainties alone.
Aims. Fundamental to any theory of high-mass star formation are gravity and turbulence. Their relative importance, which probably changes during cloud evolution, is not known. By investigating the ...spatial and density structure of the high-mass star-forming complex NGC 6334 we aim to disentangle the contributions of turbulence and gravity. Methods. We used Herschel PACS and SPIRE imaging observations from the HOBYS key programme at wavelengths of 160, 250, 350, and 500 μm to construct dust temperature and column density maps. Using probability distribution functions (PDFs) of the column density determined for the whole complex and for four distinct sub-regions (distinguished on the basis of differences in the column density, temperature, and radiation field), we characterize the density structure of the complex. We investigate the spatial structure using the Δ-variance, which probes the relative amount of structure on different size scales and traces possible energy injection mechanisms into the molecular cloud. Results. The Δ-variance analysis suggests that the significant scales of a few parsec that were found are caused by energy injection due to expanding H ii regions, which are numerous, and by the lengths of filaments seen everywhere in the complex. The column density PDFs have a lognormal shape at low densities and a clearly defined power law at high densities for all sub-regions whose slope is linked to the exponent α of an equivalent spherical density distribution. In particular with α = 2.37, the central sub-region is largly dominated by gravity, caused by individual collapsing dense cores and global collapse of a larger region. The collapse is faster than free-fall (which would lead only to α = 2) and thus requires a more dynamic scenario (external compression, flows). The column density PDFs suggest that the different sub-regions are at different evolutionary stages, especially the central sub-region, which seems to be in a more evolved stage.
Context. The role of ionization feedback on high-mass (>8 M⊙) star formation is still highly debated. Questions remain concerning the presence of nearby H II regions changes the properties of early ...high-mass star formation and whether H II regions promote or inhibit the formation of high-mass stars. Aims. To characterize the role of H II regions on the formation of high-mass stars, we study the properties of a sample of candidates high-mass starless clumps (HMSCs), of which about 90% have masses larger than 100 M⊙. These high-mass objects probably represent the earliest stages of high-mass star formation; we search if (and how) their properties are modified by the presence of an H II region. Methods. We took advantage of the recently published catalog of HMSC candidates. By cross matching the HMSCs and H II regions, we classified HMSCs into three categories: (1) the HMSCs associated with H II regions both in the position in the projected plane of the sky and in velocity; (2) HMSCs associated in the plane of the sky, but not in velocity; and (3) HMSCs far away from any H II regions in the projected sky plane. We carried out comparisons between associated and nonassociated HMSCs based on statistical analyses of multiwavelength data from infrared to radio. Results. We show that there are systematic differences of the properties of HMSCs in different environments. Statistical analyses suggest that HMSCs associated with H II regions are warmer, more luminous, more centrally-peaked and turbulent. We also clearly show, for the first time, that the ratio of bolometric luminosity to envelope mass of HMSCs (L∕M) could not be a reliable evolutionary probe for early massive star formation due to the external heating effects of the H II regions. Conclusions. We show HMSCs associated with H II regions present statistically significant differences from HMSCs far away from H II regions, especially for dust temperature and L∕M. More centrally peaked and turbulent properties of HMSCs associated with H II regions may promote the formation of high-mass stars by limiting fragmentation. High-resolution interferometric surveys toward HMSCs are crucial to reveal how H II regions impact the star formation process inside HMSCs.
Context. The ionization feedback from H II regions modifies the properties of high-mass starless clumps (HMSCs, of several hundred to a few thousand solar masses with a typical size of 0.1–1 pc), ...such as dust temperature and turbulence, on the clump scale. The question of whether the presence of H II regions modifies the core-scale (~0.025 pc) fragmentation and star formation in HMSCs remains to be explored. Aims. We aim to investigate the difference of 0.025 pc-scale fragmentation between candidate HMSCs that are strongly impacted by H II regions and less disturbed ones. We also search for evidence of mass shaping and induced star formation in the impacted candidate HMSCs. Methods. Using the ALMA 1.3 mm continuum, with a typical angular resolution of 1.3′′, we imaged eight candidate HMSCs, including four impacted by H II regions and another four situated in the quiet environment. The less-impacted candidate HMSCs are selected on the basis of their similar mass and distance compared to the impacted ones to avoid any possible bias linked to these parameters. We carried out a comparison between the two types of candidate HMSCs. We used multi-wavelength data to analyze the interaction between H II regions and the impacted candidate HMSCs. Results. A total of 51 cores were detected in eight clumps, with three to nine cores for each clump. Within our limited sample, we did not find a clear difference in the ~0.025 pc-scale fragmentation between impacted and non-impacted candidate HMSCs, even though H II regions seem to affect the spatial distribution of the fragmented cores. Both types of candidate HMSCs present a thermal fragmentation with two-level hierarchical features at the clump thermal Jeans length λJ,clumpth and 0.3λJ,clumpth. The ALMA emission morphology of the impacted candidate HMSCs AGAL010.214-00.306 and AGAL018.931-00.029 sheds light on the capacities of H II regions to shape gas and dust in their surroundings and possibly to trigger star formation at ~0.025 pc-scale in candidate HMSCs. Conclusions. The fragmentation at ~0.025 pc scale for both types of candidate HMSCs is likely to be thermal-dominant, meanwhile H II regions probably have the capacity to assist in the formation of dense structures in the impacted candidate HMSCs. Future ALMA imaging surveys covering a large number of impacted candidate HMSCs with high turbulence levels are needed to confirm the trend of fragmentation indicated in this study.
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHKS images of the Galactic H II region RCW 79. The millimetre continuum data reveal the presence of massive fragments located in a dust ...emission ring surrounding the ionized gas. The two most massive fragments are diametrically opposite each other in the ring. The near-IR data, centred on the compact H II region located at the south-eastern border of RCW 79, show the presence of an IR-bright cluster containing massive stars along with young stellar objects with near-IR excesses. A bright near- and mid-IR source is detected towards maser emissions, 1.2 pc north-east of the compact H II region centre. Additional information extracted from the Spitzer GLIMPSE survey is used to discuss the nature of the bright IR sources observed towards RCW 79. Twelve luminous Class I sources are identified towards the most massive millimetre fragments. All these facts strongly indicate that the massive-star formation observed at the border of the H II region RCW 79 has been triggered by its expansion, most probably by the collect and collapse process.
PDF
