Triggered massive-star formation on the borders of Galactic HII regions. II. 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.Comment: 14 pages, 15 figures. Accepted for publication in A&A. The images have been highly compressed for astro-ph. A version of this paper with higher-resolution figures is available at http://www.oamp.fr/matiere/rcw79.pd

Triggered massive-star formation on the borders of Galactic HII regions. III. Star formation at the periphery of Sh2-219

Context. Massive-star formation triggered by the expansion of HII regions. Aims. To understand if sequential star formation is taking place at the periphery of the HII region Sh2-219. Methods. We present 12CO(2-1) line observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta, Spain). Results. In the optical, Sh2-219 is spherically symmetric around its exciting star; furthermore it is surrounded along three quarters of its periphery by a ring of atomic hydrogen. This spherical symmetry breaks down at infrared and millimetre wavelengths. A molecular cloud of about 2000\msol lies at the southwestern border of Sh2-219, in the HI gap. Two molecular condensations, elongated along the ionization front, probably result from the interaction between the expanding HII region and the molecular cloud. In this region of interaction there lies a cluster containing many highly reddened stars, as well as a massive star exciting an ultracompact HII region. More surprisingly, the brightest parts of the molecular cloud form a `chimney', perpendicular to the ionization front. This chimney is closed at its south-west extremity by H-alpha walls, thus forming a cavity. The whole structure is 7.5 pc long. A luminous H-alpha emission-line star, lying at one end of the chimney near the ionization front, may be responsible for this structure. Confrontation of the observations with models of HII region evolution shows that Sh2-219 is probably 10^5 yr old. The age and origin of the near-IR cluster observed on the border of Sh2-219 remain unknown.Comment: 11 pages, 10 figures. To be published in A&

Spatial distribution of unidentified infrared bands and extended red emission in the compact galactic HII region Sh 152

We present visible and near IR images of the compact HII region Sh 152. Some of these images reveal the presence of Extended Red Emission (ERE) around 698 nm and emission from Unidentified Infra Red Bands (UIRBs) at 3.3 and 6.2 micron. Other images show the near infrared (7-12 micron) continuous emission of the nebula. The ERE emission is found to coincide with the ionized region and significantly differ from the UIRBs location. Also some evidence is found in favor of grains as carriers for ERE.Comment: 3 pages, 4 figures, to be published in the proceedings of the colloquium "The universe as seen by ISO" help in Paris, October 20-23, 1998 ; available in html format at http://www.obs-hp.fr/preprints.htm

HerschelHerschel SPIRE-FTS observations of RCW 120

The expansion of Galactic HII regions can trigger the formation of a new generation of stars. However, little is know about the physical conditions that prevail in these regions. We study the physical conditions that prevail in specific zones towards expanding HII regions that trace representative media such as the photodissociation region, the ionized region, and condensations with and without ongoing star formation. We use the SPIRE Fourier Transform Spectrometer (FTS) on board $Herschel$ to observe the HII region RCW 120. Continuum and lines are observed in the $190-670\,\mu$m range. Line intensities and line ratios are obtained and used as physical diagnostics of the gas. We used the Meudon PDR code and the RADEX code to derive the gas density and the radiation field at nine distinct positions including the PDR surface and regions with and without star-formation activity. For the different regions we detect the atomic lines [NII] at $205\,\mu$m and [CI] at $370$ and $609\,\mu$m, the $^{12}{\rm CO}$ ladder between the $J=4$ and $J=13$ levels and the $^{13}{\rm CO}$ ladder between the $J=5$ and $J=14$ levels, as well as CH$^{+}$ in absorption. We find gas temperatures in the range $45-250\,$K for densities of $10^4-10^6\,{\rm cm}^{-3}$, and a high column density on the order of $N_{{\rm H}}\sim10^{22}\,{\rm cm}^{-2}$ that is in agreement with dust analysis. The ubiquitousness of the atomic and CH$^{+}$ emission suggests the presence of a low-density PDR throughout RCW 120. High-excitation lines of CO indicate the presence of irradiated dense structures or small dense clumps containing young stellar objects, while we also find a less dense medium ($N_{{\rm H}}\sim10^{20}\,{\rm cm}^{-2}$) with high temperatures ($80-200\,$K).Comment: 11 pages, 11 figures, accepted by A&

Deep GeMS/GSAOI near-infrared observations of N159W in the Large Magellanic Cloud

Aims. The formation and properties of star clusters at the edge of H II regions are poorly known, partly due to limitations in angular resolution and sensitivity, which become particularly critical when dealing with extragalactic clusters. In this paper we study the stellar content and star-formation processes in the young N159W region in the Large Magellanic Cloud. Methods. We investigate the star-forming sites in N159W at unprecedented spatial resolution using JHKs-band images obtained with the GeMS/GSAOI instrument on the Gemini South telescope. The typical angular resolution of the images is of 100 mas, with a limiting magnitude in H of 22 mag (90 percent completeness). Photometry from our images is used to identify candidate young stellar objects (YSOs) in N159W. We also determine the H-band luminosity function of the star cluster at the centre of the H II region and use this to estimate its initial mass function (IMF). Results. We estimate an age of 2 + or - 1 Myr for the central cluster, with its IMF described by a power-law with an index of gamma = - 1.05 + or - 0.2 , and with a total estimated mass of 1300 solar mass. We also identify 104 candidate YSOs, which are concentrated in clumps and subclusters of stars, principally at the edges of the H II region. These clusters display signs of recent and active star-formation such as ultra-compact H II regions, and molecular outflows. This suggests that the YSOs are typically younger than the central cluster, pointing to sequential star-formation in N159W, which has probably been influenced by interactions with the expanding H II bubble

Correction of distortion for optimal image stacking in Wide Field Adaptive Optics: Application to GeMS data

The advent of Wide Field Adaptive Optics (WFAO) systems marks the beginning of a new era in high spatial resolution imaging. The newly commissioned Gemini South Multi-Conjugate Adaptive Optics System (GeMS) combined with the infrared camera Gemini South Adaptive Optics Imager (GSAOI), delivers quasi diffraction-limited images over a field of 2 arc-minutes across. However, despite this excellent performance, some variable residues still limit the quality of the analyses. In particular, distortions severely affect GSAOI and become a critical issue for high-precision astrometry and photometry. In this paper, we investigate an optimal way to correct for the distortion following an inverse problem approach. Formalism as well as applications on GeMS data are presented.Comment: 10 pages, 6 figure

The molecular complex associated with the Galactic HII region Sh2-90: a possible site of triggered star formation

We investigate the star formation activity in the molecular complex associated with the Galactic HII region Sh2-90, using radio-continuum maps obtained at 1280 MHz and 610 MHz, Herschel Hi-GAL observations at 70 -- 500 microns, and deep near-infrared observation at JHK bands, along with Spitzer observations. Sh2-90 presents a bubble morphology in the mid-IR (size ~ 0.9 pc x 1.6 pc). Radio observations suggest it is an evolved HII region with an electron density ~ 144 cm^-3, emission measure ~ 6.7 x 10^4 cm^-6 pc and a ionized mass ~ 55 Msun. From Hi-GAL observations it is found that the HII region is part of an elongated extended molecular cloud (size ~ 5.6 pc x 9.7 pc, H_2 column density >= 3 x 10^21 cm^-2 and dust temperature 18 -- 27 K) of total mass >= 1 x 10^4 Msun. We identify the ionizing cluster of Sh2-90, the main exciting star being an O8--O9 V star. Five cold dust clumps (mass ~ 8 -- 95 Msun), four mid-IR blobs around B stars, and a compact HII region are found at the edge of the bubble.The velocity information derived from CO (J=3-2) data cubes suggests that most of them are associated with the Sh2-90 region. 129 YSOs are identified (Class I, Class II, and near-IR excess sources). The majority of the YSOs are low mass (<= 3 Msun) sources and they are distributed mostly in the regions of high column density. Four candidate Class 0/I MYSOs have been found; they will possibly evolve to stars of mass >= 15 Msun. We suggest multi-generation star formation is present in the complex. From the evidences of interaction, the time scales involved and the evolutionary status of stellar/protostellar sources, we argue that the star formation at the immediate border/edges of Sh2-90 might have been triggered by the expanding HII region. However, several young sources in this complex are probably formed by some other processes.Comment: 22 pages, 22 figures, accepted for publication in Astronomy and Astrophysic

Mopra CO Observations of the Bubble HII Region RCW120

We use the Mopra radio telescope to test for expansion of the molecular gas associated with the bubble HII region RCW120. A ring, or bubble, morphology is common for Galactic HII regions, but the three-dimensional geometry of such objects is still unclear. Detected near- and far-side expansion of the associated molecular material would be consistent with a three-dimensional spherical object. We map the $J = 1\rightarrow 0$ transitions of $^{12}$CO, $^{13}$CO, C$^{18}$O, and C$^{17}$O, and detect emission from all isotopologues. We do not detect the $0_0\rightarrow 1_{-1} E$ masing lines of CH$_3$OH at 108.8939 GHz. The strongest CO emission is from the photodissociation region (PDR), and there is a deficit of emission toward the bubble interior. We find no evidence for expansion of the molecular material associated with RCW120 and therefore can make no claims about its geometry. The lack of detected expansion is roughly in agreement with models for the time-evolution of an HII region like RCW120, and is consistent with an expansion speed of $< 1.5\, {\rm km\, s^{-1}}$. Single-position CO spectra show signatures of expansion, which underscores the importance of mapped spectra for such work. Dust temperature enhancements outside the PDR of RCW120 coincide with a deficit of emission in CO, confirming that these temperature enhancements are due to holes in the RCW120 PDR. H$\alpha$ emission shows that RCW120 is leaking $\sim5\%$ of the ionizing photons into the interstellar medium (ISM) through PDR holes at the locations of the temperature enhancements. H-alpha emission also shows a diffuse "halo" from leaked photons not associated with discrete holes in the PDR. Overall $25\pm10\%$ of all ionizing photons are leaking into the nearby ISM.Comment: 35 pages, 14 figures. Accepted to Ap

Star Formation Towards The Galactic H Ii Region Rcw 120 - Herschel Observations Of Compact Sources

The expansion of H ii regions can trigger the formation of stars. An overdensity of young stellar objects is observed at the edges of H ii regions but the mechanisms that give rise to this phenomenon are not clearly identified. Moreover, it is difficult to establish a causal link between H ii -region expansion and the star formation observed at the edges of these regions. A clear age gradient observed in the spatial distribution of young sources in the surrounding might be a strong argument in favor of triggering