Guido Garay

ATLASGAL - The APEX telescope large area survey of the galaxy at 870 μm

Context. Thanks to its excellent 5100 m high site in Chajnantor, the Atacama Pathfinder Experiment (APEX) systematically explore s the southern sky at submillimeter wavelengths, both in continuum and in spectral line emission. Studying continuum emission from interstellar dust is essential to locate the highest densit y regions in the interstellar medium, and to derive their masses, column densities, density structures, and larger scale morpholog ies. In particular, the early stages of (massive) star forma tion are still quite mysterious: only small samples of high-mass proto-stellar or young stellar objects have been studied in detail so far. Aims. Our goal is to produce a large scale, systematic database of massive pre- and proto-stellar clumps in the Galaxy, in order to better understand how and under what conditions star formation takes place. Only a systematic survey of the Galactic Plane can provide the statistical basis for unbiased studies. A well characteriz ed sample of Galactic star-forming sites will deliver an evolutionary sequence and a mass function of high-mass star-forming clumps. Such a systematic survey at submillimeter wavelengths also represents a pioneering work in preparation for Herschel and ALMA. Methods. The APEX telescope is ideally located to observe the inner Milky Way. The recently commissioned Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 µm, with a beam of 19. ′′ 2. Taking advantage of its large field of view (11. ′ 4) and excellent sensitivity, we have started an unbiased survey of the whole Galactic Plane accessible to APEX, with a typical noise level of 50‐70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). Results. As a first step, we have covered ∼95 deg 2 of the Galactic Plane. These data reveal∼6000 compact sources brighter than 0.25 Jy, or 63 sources per square degree, as well as extended structures, many of them filamentary. About two thirds of the c ompact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or proto-clusters. Other compact sources harbor hot cores, compact Hii regions or young embedded clusters, thus tracing more evolved stages after star formation has occurred. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M⊙. In this first introductory paper, we show preliminary resul ts from these ongoing observations, and discuss the mid- and long-term perspectives of the survey.

ATLASGAL - The APEX Telescope Large Area Survey of the Galaxy at 870 microns

Context. Thanks to its excellent 5100 m high site in Chajnantor, the Atacama Pathfinder Experiment (APEX) systematically explore s the southern sky at submillimeter wavelengths, both in continuum and in spectral line emission. Studying continuum emission from interstellar dust is essential to locate the highest densit y regions in the interstellar medium, and to derive their masses, column densities, density structures, and larger scale morpholog ies. In particular, the early stages of (massive) star forma tion are still quite mysterious: only small samples of high-mass proto-stellar or young stellar objects have been studied in detail so far. Aims. Our goal is to produce a large scale, systematic database of massive pre- and proto-stellar clumps in the Galaxy, in order to better understand how and under what conditions star formation takes place. Only a systematic survey of the Galactic Plane can provide the statistical basis for unbiased studies. A well characteriz ed sample of Galactic star-forming sites will deliver an evolutionary sequence and a mass function of high-mass star-forming clumps. Such a systematic survey at submillimeter wavelengths also represents a pioneering work in preparation for Herschel and ALMA. Methods. The APEX telescope is ideally located to observe the inner Milky Way. The recently commissioned Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 µm, with a beam of 19. ′′ 2. Taking advantage of its large field of view (11. ′ 4) and excellent sensitivity, we have started an unbiased survey of the whole Galactic Plane accessible to APEX, with a typical noise level of 50‐70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). Results. As a first step, we have covered ∼95 deg 2 of the Galactic Plane. These data reveal∼6000 compact sources brighter than 0.25 Jy, or 63 sources per square degree, as well as extended structures, many of them filamentary. About two thirds of the c ompact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or proto-clusters. Other compact sources harbor hot cores, compact Hii regions or young embedded clusters, thus tracing more evolved stages after star formation has occurred. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M⊙. In this first introductory paper, we show preliminary resul ts from these ongoing observations, and discuss the mid- and long-term perspectives of the survey.

A Search for Infall Motions toward Nearby Young Stellar Objects

We report observations of 47 candidate protostars in two optically thick lines [H2CO (212-111) and CS (2-1)] and one optically thin line [N2H+ (1-0)] using the IRAM 30 m, SEST 15 m, and Haystack 37 m radio telescopes. The sources were selected for the redness of their spectra (Tbol < 200 K) and their near distance (d < 400 pc). Most of the sources have asymmetric, optically thick lines. The observed distribution of velocity differences, ?V = (Vthick - Vthin)/?Vthin, is skewed toward negative (blueshifted) velocities for both the H2CO and CS samples. This excess is much more significant for class 0 than for class I sources, suggesting that we detect infall motions toward class 0 and not toward class I sources. This indicates a difference in the physical conditions in the circumstellar envelopes around class I and class 0 sources, but it does not rule out the presence of infall onto class I sources by, for example, lower opacity gas. Bipolar outflows alone, or rotation alone, cannot reproduce these statistics if the sample of sources has randomly oriented symmetry axes. We identify 15 spectroscopic infall candidates, six of which are new. Most of these infall candidates have primarily turbulent rather than thermal motions and are associated with clusters rather than being isolated.

Massive Stars : Their Environment and Formation

Cloud environment is thought to play a critical role in determining the mechanism of formation of massive stars. In this contribution we review the physical characteristics of the environment around recently formed massive stars. Particular emphasis is given to recent high angular resolution obser- vations which have improved our knowledge of the physical conditions and kinematics of compact regions of ionized gas and of dense and hot molecular cores associated with luminous O and B stars. We will show that this large body of data, gathered during the last decade, has allowed signiÐcant progress in the understanding of the physical processes that take place during the formation and early evolution of massive stars.

SIMBA survey of southern high-mass star forming regions. I. Physical parameters of the 1.2-mm/IRAS sources

We report the results of a 1.2 mm continuum emission survey toward 146 IRAS sources thought to harbour high- mass star forming regions. The sources have FIR colors typical of UCHII regions and were detected in the CS(2 → 1) line survey of Bronfman et al. (1996). Regions of 15 � × 10 � , centered on each IRAS source, were mapped with an angular resolution of ∼24 �� , using the SIMBA array on the SEST telescope. 1.2 mm emission was detected toward all IRAS sources. We find that the dust cores associated with these sources have typical sizes of 0.4 pc and masses of 5 × 10 3 M� . Dust temperatures and luminosities, derived from the SED, are typically 32 K and 2.3 × 10 5 L� .

Millimetre continuum observations of southern massive star formation regions. I. SIMBA observations of cold cores

We report the results of a 1.2-mm continuum emission survey toward 131 star-forming complexes suspected of undergoing massive star formation. These regions have previously been identified as harbouring a methanol maser and/or a radio continuum source [ultracompact (UC) H II region], the presence of which is in most instances indicative of massive star formation. The 1.2-mm emission was mapped using the SIMBA instrument on the 15-m Swedish ESO Submillimetre Telescope (SEST). Emission is detected toward all of the methanol maser and UC H II regions targeted, as well as towards 20 others lying within the fields mapped, implying that these objects are associated with cold, deeply embedded objects. Interestingly, there are also 20 methanol maser sites and nine UC H II regions within the fields mapped which are devoid of millimetre continuum emission. In addition to the maser and UC H II regions detected, we have also identified 253 other sources within the SIMBA maps. All of these (253) are new sources, detected solely from their millimetre continuum emission. These ‘mm-only’ cores are devoid of the traditional indicators of massive star formation, (i.e. methanol/OH maser, UC H II regions or IRAS point sources). At least 45 per cent of these mm-only cores are also without mid-infrared Mid-course Space Experiment (MSX) emission. The ‘mm-only’ core may be an entirely new class of source that represents an earlier stage in the evolution of massive stars, prior to the onset of methanol maser emission. Or, they may harbour protoclusters which do not contain any high-mass stars (i.e. below the H II region limit). In total, 404 sources are detected, representing four classes of sources which are distinguished by the presence of the different combination of associated tracer/s. Their masses, estimated assuming a dust temperature of 20 K and adopting kinematic distances, range from 0.5 × 10 1 to 3.7 × 10 4 M� , with an average mass for the sample of 1.5 × 10 3 M� . The H2 number density (nH2 )o fthe source sample ranges from 1.4 × 10 3 to 1.9 × 10 6 cm −3 , with an average of 8.7 × 10 4 cm −3 . The average radius of the sample is 0.5 pc. The visual extinction ranges from 10 to 500 mag with an average of 80 mag, which implies a high degree of embedding. The surface density (� )v aries from 0.2 to 18.0 kg m −2 with an average of 2.8 kg m −2 . Analysis of the millimetre-only sources shows that they are less massive ( ¯

VLA Observations of Strong IRAS Point Sources Associated with Compact H II Regions

We present multifrequency radio continuum observations made with the Very Large Array of a sample of 16 strong IRAS point sources associated with compact H II regions. Twelve sources were observed at 1.5, 4.9, and 15.0 GHz, with angular resolution of ∼15″, 5″, and 3″, respectively. Three were observed at 1.5 and 4.9 GHz, and one at 1.5 GHz. The radio continuum brightness distribution was resolved for all sources. Nine objects show simple mophologies: core-halo (2), shell (3), cometary (3), and bipolar (1), consistent with their being excited by a single star

Compact continuum radio sources in the Orion Nebula

The Orion Nebula was observed with the VLA in order to search for radio emission from compact H II regions indicative of embedded OB stars or from winds associated with pre-main sequence, low-mass stars. Fourteen of the 21 detected radio sources are within 30 arcsec of Omega 1 Orionis C; 13 of these objects are probably neutral condensations surrounded by ionized envelopes that are excited by the star. If the temperature of the ionized envelopes is 10,000 K and their electron densities decrease as the square of the distance from the core center, then a typical neutral condensation has a radius of 10 to the 15th cm and a peak electron density of 400,000/cu cm. Seven sources are in or near the Orion molecular cloud. Four of the sources have optical counterparts. Two are highly variable radio sources associated with X-ray sources, and two have radio spectra indicative of thermal emission. Two of the three optically invisible sources have radio emission likely to arise in a dense ionized envelope surrounding and excited by an early B-type star. 46 references.

Large Proper Motions in the Jet of the High-Mass YSO Cepheus A HW2

Using high angular resolution (~025-005) Very Large Array (VLA) observations made at 3.6 cm, 1.3 cm, and 7 mm during the period 1991-2004, we report the detection of large proper motions in the components of the radio continuum jet associated with the high-mass young stellar object (YSO) HW2 in the star-forming region Cepheus A. The relative proper motions observed for the two main components of the outflow, moving away from the central source in nearly opposite directions, are of the order of 140 mas yr-1, or ~480 km s-1 at a distance of 725 pc. The proper motions observed in the northeast and southwest lobes are not completely antiparallel, and the central elongated source seems to be changing orientation. We discuss possible scenarios to account for these and other observed characteristics. We also report the detection of a 7 mm compact continuum condensation of emission near the center of the thermal radio continuum jet, which we propose as the location of the exciting star.

Star-forming Protoclusters Associated with Methanol Masers

We present a multiwavelength study of five methanol maser sites which are not directly associated with a strong (>100 mJy) radio continuum source: G 31.28+0.06, G 59.78+0.06, G 173.49+2.42 (S231, S233IR), G 188.95+0.89 (S252, AFGL5180) and G 192.60-0.05 (S255IR). These radio-quiet methanol maser sites are often interpreted as precursors of ultra- compact H  regions or massive protostar sites. In this work, the environment of methanol masers is probed from mid-IR to millimetre wavelengths at angular resolutions of 8 �� −34 �� . Spectral energy distribution (SED) diagrams for each site are pre- sented, together with mass and luminosity estimates. Each radio-quiet maser site is always associated with a massive (>50 M� ), deeply embedded (Av > 40 mag) and very luminous (>10 4 L� ) molecular clump, with Ltotal ∝ M 0.75 gas . These physical properties characterise massive star-forming clumps in earlier evolutionary phases than H  regions. In addition, colder gas clumps seen only at mm-wavelengths are also found near the methanol maser sites. These colder clumps may represent an even earlier phase of massive star formation. These results suggest an evolutionary sequence for massive star formation from a cold clump, seen only at mm wavelengths, evolving to a hot molecular core with a two-component SED with peaks at far-IR and mid-IR wave- lengths, to an (ultra-compact) H  region. Alternatively, the cold clumps might be clusters of low-mass YSOs, in formation near the massive star-forming clusters. Finally, the values of the dust grain emissivity index (β) range between 1.6 and 1.9.