267 research outputs found
Triggered star formation on the borders of the Galactic HII region RCW 120
To investigate the process of star formation triggered by the expansion of an
HII region, we present a multi-wavelength analysis of the Galactic HII region
RCW 120 and its surroundings. The collect and collapse model predicts that the
layer of gas and dust accumulated between the ionization and shock fronts
during the expansion of the HII region collapses and forms dense fragments,
giving rise to potential sites of massive-star formation. The aim of our study
is to look for such massive fragments and massive young stars on the borders of
RCW 120. We mapped the RCW 120 region in the cold dust continuum emission at
1.2 mm to search for these fragments. We supplemented this study with the
available near- (2MASS) and mid-IR (GLIMPSE) data to locate the IR sources
observed towards this region and to analyse their properties. We then compared
the observational results with the predictions of Hosokawa & Inutsuka's model
(2005, 2006). At 1.2 mm we detected eight fragments towards this region, five
located on its borders. The largest fragment has a mass of about 370 Msun.
Class I and Class II young stellar objects are detected all over the region,
with some observed far from the ionization front. This result emphasises the
possible importance distant interactions between the radiation, escaping from
the ionized region, and the surrounding medium
Herschel Observations Of The Galactic H Ii Region Rcw 79
Triggered star formation around H II regions could be an important process. The Galactic H II region RCW79 is a prototypical object for triggered highâmass star formation. Aims. We aim to obtain a census of the young stellar population observed at the edges of the H II region and to determine the properties of the young sources in order to characterize the star formation processes that take place at the edges of this ionized region. Methods. We take advantage of Herschel data from the surveys HOBYS, `` Evolution of Interstellar Dust\u27\u27, and HiâGal to extract compact sources. We use the algorithm getsources. We complement the Herschel data with archival 2MASS, Spitzer, and WISE data to determine the physical parameters of the sources (e. g., envelope mass, dust temperature, and luminosity) by fitting the spectral energy distribution. Results. We created the dust temperature and column density maps along with the column density probability distribution function (PDF) for the entire RCW79 region. We obtained a sample of 50 compact sources in this region, 96% of which are situated in the ionizationâcompressed layer of cold and dense gas that is characterized by the column density PDF with a doubleâpeaked lognormal distribution. The 50 sources have sizes of similar to 0.1â0.4 pc with a typical value of similar to 0 : 2 pc, temperatures of similar to 11â26 K, envelope masses of similar to 6 760 Mâcircle dot, densities of similar to 0.1 44 +/â 10(5) cm(3), and luminosities of similar to 19â12712 Lâcircle dot. The sources are classified into 16 class 0, 19 intermediate, and 15 class I objects. Their distribution follows the evolutionary tracks in the diagram of bolometric luminosity versus envelope mass (LâbolâMâenv) well. A mass threshold of 140 M fi, determined from the LâbolâMâenv diagram, yields 12 candidate massive dense cores that may form highâmass stars. The core formation efficiency (CFE) for the 8 massive condensations shows an increasing trend of the CFE with density. This suggests that the denser the condensation, the higher the fraction of its mass transformation into dense cores, as previously observed in other highâmass starâforming regions
Stellar Feedback in the Star Formation-Gas Density Relation: Comparison between Simulations and Observations
Context. The impact of stellar feedback on the Kennicutt-Schmidt law (KS
law), which relates star formation rate (SFR) to surface gas density, is a
topic of ongoing debate. The interpretation of individual cloud observations is
challenging due to the various processes at play simultaneously and inherent
biases. Therefore, a numerical investigation is necessary to understand the
role of stellar feedback and identify observable signatures.
Aims. We investigate the role of stellar feedback on the KS law, aiming to
identify distinct signatures that can be observed and analysed.
Methods. We analyse MHD numerical simulations of a cloud
evolving under different feedback prescriptions. The set of simulations
contains four types of feedback: with only protostellar jets, with ionising
radiation from massive stars , with both of them and without
any stellar feedback. To compare these simulations with the existing
observational results, we analyse their evolution by adopting the same
techniques applied in observational studies. Then, we simulate how the same
analyses would change if the data were affected by typical observational
biases.
Conclusions. The presence of stellar feedback strongly influences the KS
relation and the star formation efficiency per free-fall time
(). Its impact is primarily governed by its influence on
the cloud's structure. Although the measured in our
clouds results to be higher than what is usually observed in real clouds, upon
applying prescriptions to mimic observational biases we recover good agreement
with the expected values. Therefore, we can infer that observations tend to
underestimate the total SFR. Moreover, this likely indicates that the physics
included in our simulations is sufficient to reproduce the basic mechanisms
contributing to set .Comment: 16 pages, 13 figures, accepted for publication to A&
Triggered massive-star formation on the borders of Galactic HII regions Evidence for the collect and collapse process around RCW 79
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of the Galactic HII 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 HII 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 HII region centre. Additional information, extracted from the Spitzer GLIMPSE survey, are 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 HII region RCW 79 has been triggered by its expansion, most probably by the collect and collapse process
High-mass Starless Clumps in the inner Galactic Plane: the Sample and Dust Properties
We report a sample of 463 high-mass starless clump (HMSC) candidates within
and . This sample has been singled out from
10861 ATLASGAL clumps. All of these sources are not associated with any known
star-forming activities collected in SIMBAD and young stellar objects
identified using color-based criteria. We also make sure that the HMSC
candidates have neither point sources at 24 and 70 \micron~nor strong extended
emission at 24 m. Most of the identified HMSCs are infrared (
m) dark and some are even dark at 70 m. Their distribution shows
crowding in Galactic spiral arms and toward the Galactic center and some
well-known star-forming complexes. Many HMSCs are associated with large-scale
filaments. Some basic parameters were attained from column density and dust
temperature maps constructed via fitting far-infrared and submillimeter
continuum data to modified blackbodies. The HMSC candidates have sizes, masses,
and densities similar to clumps associated with Class II methanol masers and
HII regions, suggesting they will evolve into star-forming clumps. More than
90% of the HMSC candidates have densities above some proposed thresholds for
forming high-mass stars. With dust temperatures and luminosity-to-mass ratios
significantly lower than that for star-forming sources, the HMSC candidates are
externally heated and genuinely at very early stages of high-mass star
formation. Twenty sources with equivalent radius pc and
mass surface density g cm could be possible high-mass
starless cores. Further investigations toward these HMSCs would undoubtedly
shed light on comprehensively understanding the birth of high-mass stars.Comment: 16 pages, 15 figures, and 5 tables. Accepted for publication in ApJS.
FITS images for the far-IR to sub-mm data, H2 column density and dust
temperature maps of all the HMSC candidates are available at https:
//yuanjinghua.github.io/hmscs.html. Codes used for this work are publicly
available from https://github.com/yuanjinghua/HMSCs_ca
Interactions of the Infrared bubble N4 with the surroundings
The physical mechanisms that induce the transformation of a certain mass of
gas in new stars are far from being well understood. Infrared bubbles
associated with HII regions have been considered to be good samples of
investigating triggered star formation. In this paper we report on the
investigation of the dust properties of the infrared bubble N4 around the HII
region G11.898+0.747, analyzing its interaction with its surroundings and star
formation histories therein, with the aim of determining the possibility of
star formation triggered by the expansion of the bubble. Using Herschel PACS
and SPIRE images with a wide wavelength coverage, we reveal the dust properties
over the entire bubble. Meanwhile, we are able to identify six dust clumps
surrounding the bubble, with a mean size of 0.50 pc, temperature of about 22 K,
mean column density of 1.7 cm, mean volume density of
about 4.4 cm, and a mean mass of 320 . In
addition, from PAH emission seen at 8 m, free-free emission detected at 20
cm and a probability density function in special regions, we could identify
clear signatures of the influence of the HII region on the surroundings. There
are hints of star formation, though further investigation is required to
demonstrate that N4 is the triggering source.Comment: Accepted by ApJ (16 pages, 11 figures, 9 tables
Calibration of the AKARI Far-Infrared Imaging Fourier Transform Spectrometer
The Far-Infrared Surveyor (FIS) onboard the AKARI satellite has a
spectroscopic capability provided by a Fourier transform spectrometer
(FIS-FTS). FIS-FTS is the first space-borne imaging FTS dedicated to
far-infrared astronomical observations. We describe the calibration process of
the FIS-FTS and discuss its accuracy and reliability. The calibration is based
on the observational data of bright astronomical sources as well as two
instrumental sources. We have compared the FIS-FTS spectra with the spectra
obtained from the Long Wavelength Spectrometer (LWS) of the Infrared Space
Observatory (ISO) having a similar spectral coverage. The present calibration
method accurately reproduces the spectra of several solar system objects having
a reliable spectral model. Under this condition the relative uncertainty of the
calibration of the continuum is estimated to be 15% for SW, 10% for
70-85 cm^(-1) of LW, and 20% for 60-70 cm^(-1) of LW; and the absolute
uncertainty is estimated to be +35/-55% for SW, +35/-55% for 70-85 cm^(-1) of
LW, and +40/-60% for 60-70 cm^(-1) of LW. These values are confirmed by
comparison with theoretical models and previous observations by the ISO/LWS.Comment: 22 pages, 10 figure
Star formation in the filament of S254-S258 OB complex: a cluster in the process of making
International audienceInfrared Dark Clouds (IRDCs) are ideal laboratories to study the initial processes of high-mass star and star cluster formation. We investigated star formation activity of an unexplored filamentary dark cloud (size ⌠5.7 pc Ă 1.9 pc), which itself is part of a large filament (⌠20 pc) located in the S254-S258 OB complex at a distance of 2.5 kpc. Using Multi-band Imaging Photometer (MIPS) Spitzer 24 ”m data, we uncover 49 sources with signal-to-noise ratio greater than 5. We identified 45 sources as candidate young stellar objects (YSOs) of Class I, Flat-spectrum, and Class II nature. Additional 17 candidate YSOs (9 Class I and 8 Class II) are also identified using JHK and Wide-field Infrared Survey Explorer (WISE) photometry. We find that the protostar to Class II sources ratio (⌠2) and the protostar fraction (⌠70 %) of the region are high. When the protostar fraction compared to other young clusters, it suggests that the star formation in the dark cloud was possibly started only 1 Myr ago. Combining the near-infrared photometry of the YSO candidates with the theoretical evolutionary models, we infer that most of the candidate YSOs formed in the dark cloud are low-mass (< 2 M) in nature. We examine the spatial distribution of the YSOs and find that majority of them are linearly aligned along the highest column density line (N(H 2)⌠1 Ă 10 22 cm â2) of the dark cloud along its long axis at mean nearest neighbor separation of ⌠0.2 pc. Using observed properties of the YSOs, physical conditions of the cloud and a simple cylindrical model, we explore the possible star formation process of this filamentary dark cloud and suggest that gravitational fragmentation within the filament should have played a dominant role in the formation of the YSOs. From the total mass of the YSOs, gaseous mass associated with the dark cloud, and surrounding environment, we infer that the region is presently forming stars at an efficiency ⌠3% and a rate ⌠30 M Myr â1 , and may emerge to a richer cluster
The role of Galactic H iiregions in the formation of filaments High-resolution submilimeter imaging of RCW 120 with ArTĂ©MiS
Context. Massive stars and their associated ionized (H ii) regions could play a key role in the formation and evolution of filaments that host star formation. However, the properties of filaments that interact with H ii regions are still poorly known.
Aims. To investigate the impact of H ii regions on the formation of filaments, we imaged the Galactic H ii region RCW 120 and its surroundings where active star formation takes place and where the role of ionization feedback on the star formation process has already been studied.
Methods. We used the large-format bolometer camera ArTéMiS on the APEX telescope and combined the high-resolution ArTéMiS data at 350 ”m and 450 ”m with Herschel-SPIRE/HOBYS data at 350 and 500 ”m to ensure good sensitivity to a broad range of spatial scales. This allowed us to study the dense gas distribution around RCW 120 with a resolution of 800 or 0.05 pc at a distance of 1.34 kpc.
Results. Our study allows us to trace the median radial intensity profile of the dense shell of RCW 120. This profile is asymmetric, indicating a clear compression from the H ii region on the inner part of the shell. The profile is observed to be similarly asymmetric on both lateral sides of the shell, indicating a homogeneous compression over the surface. On the contrary, the profile analysis of a radial filament associated with the shell, but located outside of it, reveals a symmetric profile, suggesting that the compression from the ionized region is limited to the dense shell. The mean intensity profile of the internal part of the shell is well fitted by a Plummerlike profile with a deconvolved Gaussian full width at half maximum (FWHM) of 0.09 pc, as observed for filaments in low-mass star-forming regions.
Conclusions. Using ArTĂ©MiS data combined with Herschel-SPIRE data, we found evidence for compression from the inner part of the RCW 120 ionized region on the surrounding dense shell. This compression is accompanied with a significant (factor 5) increase of the local column density. This study suggests that compression exerted by H ii regions may play a key role in the formation of filaments and may further act on their hosted star formation. ArTĂ©MiS data also suggest that RCW 120 might be a 3D ring, rather than a spherical structure
Unveiling the Importance of Magnetic Fields in the Evolution of Dense Clumps Formed at the Waist of Bipolar H ii Regions: A Case Study of Sh 2-201 with JCMT SCUBA-2/POL-2
Abstract: We present the properties of magnetic fields (B fields) in two clumps (clump 1 and clump 2), located at the waist of the bipolar H ii region Sh 2-201, based on James Clerk Maxwell Telescope SCUBA-2/POL-2 observations of 850 ÎŒm polarized dust emission. We find that B fields in the direction of the clumps are bent and compressed, showing bow-like morphologies, which we attribute to the feedback effect of the H ii region on the surface of the clumps. Using the modified DavisâChandrasekharâFermi method, we estimate B-field strengths of 266 and 65 ÎŒG for clump 1 and clump 2, respectively. From virial analyses and critical mass ratio estimates, we argue that clump 1 is gravitationally bound and could be undergoing collapse, whereas clump 2 is unbound and stable. We hypothesize that the interplay of the thermal pressure imparted by the H ii region, the B-field morphologies, and the various internal pressures of the clumps (such as magnetic, turbulent, and gas thermal pressures) has the following consequences: (a) formation of clumps at the waist of the H ii region; (b) progressive compression and enhancement of the B fields in the clumps; (c) stronger B fields that will shield the clumps from erosion by the H ii region and cause pressure equilibrium between the clumps and the H ii region, thereby allowing expanding ionization fronts to blow away from the filament ridge, forming bipolar H ii regions; and (d) stronger B fields and turbulence that will be able to stabilize the clumps. A study of a larger sample of bipolar H ii regions would help to determine whether our hypotheses are widely applicable
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