Ed Churchwell

The Spitzer/GLIMPSE Surveys: A New View of the Milky Way

A brief description is given of the GLIMPSE surveys, including the areas surveyed, sensitivity limits, and products. The primary motivations for this review are to describe some of the main scientific results enabled by the GLIMPSE surveys and to note potential future applications of the GLIMPSE catalogs and images. In particular, we discuss contributions to our understanding of star formation and early evolution, the interstellar medium, galactic structure, and evolved stars. Infrared dark clouds (IRDCs), young stellar objects (YSOs), and infrared bubbles/H II regions are discussed in some detail. A probable triggered star formation associated with expanding infrared bubbles is briefly mentioned. The distribution and morphologies of dust and polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium are discussed. Examples are shown from GLIMPSE images of bow shocks, pillars (elephant trunks), and instabilities in massive star-formation regions. The infrared extinction law of diffuse interstellar dust is discussed. The large-scale structure of the Galaxy has been traced by red-clump giants using the GLIMPSE point-source catalog to reveal the radius and orientation of the central bar, the stellar radial scale length, an obvious increase in star counts toward the tangency to the Scutum-Centaurus spiral arm, the lack of an obvious tangency from star counts toward the Sagittarius spiral arm, and a sharp increase in star counts toward the nuclear bulge. Recent results on evolved stars and some serendipitous discoveries are mentioned. More than 70 refereed papers have been published based on GLIMPSE data as of 2008 November.

H I Absorption toward Ultracompact H II Regions: Distances and Galactic Structure

The Very Large Array was used to observe H I absorption against 20 compact and ultracompact H II (UC H II) regions, thereby resolving the kinematic distance ambiguity to these sources. Combining these results with other sources whose distances were resolved through H I absorption, the half-width at half-maximum of the distribution of source height above and below the Galactic plane is found to be 35 ± 2 pc within the solar circle. The vertical height distribution is used to investigate the predictive accuracy of the Galactic latitude in resolving the kinematic distance ambiguity to UC H II regions without the need for H I observations. For a large difference between far and near kinematic distances, high accuracy can be achieved by choosing the kinematic distance closer to 1.84|b|-1 kpc, where b is the Galactic latitude in degrees.

Spitzer survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) IV: dust properties in the interstellar medium

The goal of this paper is to present the results of a preliminary analysis of the extended infrared (IR) emission by dust in the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). We combine Spitzer Surveying the Agents of Galaxy Evolution (SAGE) and Infrared Astronomical Satellite (IRAS) data and correlate the infrared emission with gas tracers of H I, CO, and Hα. We present a global analysis of the infrared emission as well as detailed modeling of the spectral energy distribution (SED) of a few selected regions. Extended emission by dust associated with the neutral, molecular, and diffuse ionized phases of the ISM is detected at all IR bands from 3.6 μm to 160 μm. The relative abundance of the various dust species appears quite similar to that in the Milky Way (MW) in all the regions we have modeled. We construct maps of the temperature of large dust grains. The temperature map shows variations in the range 12.1-34.7 K, with a systematic gradient from the inner to outer regions, tracing the general distribution of massive stars and individual H II regions as well as showing warmer dust in the stellar bar. This map is used to derive the far-infrared (FIR) optical depth of large dust grains. We find two main departures in the LMC with respect to expectations based on the MW: (1) excess mid-infrared (MIR) emission near 70 μm, referred to as the 70 μm excess, and (2) departures from linear correlation between the FIR optical depth and the gas column density, which we refer to as FIR excess emission. The 70 μm excess increases gradually from the MW to the LMC to the Small Magellanic Cloud (SMC), suggesting evolution with decreasing metallicity. The excess is associated with the neutral and diffuse ionized gas, with the strongest excess region located in a loop structure next to 30 Dor. We show that the 70 μm excess can be explained by a modification of the size distribution of very small grains with respect to that in the MW, and a corresponding mass increase of ≃13% of the total dust mass in selected regions. The most likely explanation is that the 70 μm excess is due to the production of large very small grains (VSG) through erosion of larger grains in the diffuse medium. This FIR excess could be due to intrinsic variations of the dust/gas ratio, which would then vary from 4.6 to 2.3 times lower than the MW values across the LMC, but X_(CO) values derived from the IR emission would then be about three times lower than those derived from the Virial analysis of the CO data. We also investigate the possibility that the FIR excess is associated with an additional gas component undetected in the available gas tracers. Assuming a constant dust abundance in all ISM phases, the additional gas component would have twice the known H I mass. We show that it is plausible that the FIR excess is due to cold atomic gas that is optically thick in the 21 cm line, while the contribution by a pure H_2 phase with no CO emission remains a possible explanation.

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

AN INTRODUCTION TO THE CHANDRA CARINA COMPLEX PROJECT

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant H II regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60 ks pointings using the Chandra X-ray Observatorys Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carinas regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of > 14,000 X-ray point sources;> 9800 of them have multiwavelength counterparts. Using Chandras unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carinas massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this special issue and that present detailed catalogs, methods, and science results.

Massive Star Formation

One of the most durable problems in all of astrophysics is how stars are formed. Since the time of LaPlace astromomers have postulated numerous scenerios, but to the present this problem has resisted solution. This is especially true for massive stars. Massive star formation (MSF) has received much less observational and theoretical attention than low mass star formation and as a consequence much less is known about its physics, indeed it could be argued that we know too little to ask the right questions. Some, however, are obvious. For example, the mechanism(s) of cluster formation is still a mystery. In particular, what determines the IMF? What are the roles of fragmentation, differential galactic rotation, and turbulence in regulating the rate, and possibly the IMF, of star formation in molecular clouds? Are bipolar outflows the mechanism for sheding angular momentum from accretion disks? How does matter actually get from an accretion disk onto a protostar? Are massive stars formed entirely by accretion processes or a combination of accretion and stellar mergers? When are stellar winds established, what is their relationship to bipolar outflows, and what role do they play in the formation process?

Spitzer Space Telescope observations of the Carina nebula: the steady march of feedback-driven star formation

We report the first results of imaging the Carina Nebula (NGC 3372) with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope, providing a photometry catalog of over 44,000 point sources as well as a catalog of over 900 candidate young stellar objects (YSOs) based on fits to their spectral energy distributions (SEDs). We discuss several aspects of the extended emission, including the structure of dozens of dust pillars that result when a clumpy molecular cloud is shredded by feedback from massive stars. There are surprisingly few of the “extended green objects” (EGOs) that are normally taken as signposts of outflow activity in Spitzer data, and not one of the dozens of Herbig-Haro jets detected optically are seen as EGOs. EGOs are apparently poor tracers of outflow activity in strongly irradiated environments, due to the e!ects of massive star feedback. A population of “extende dr ed objects” tends to be found around late O-type and early B-type stars, some with clear bow-shock morphology. These are dusty shocks where stellar winds collide with photoevaporative flows o! nearby clouds. Finally, the relative distributions of O-type s tars, small star clusters, and sub-clusters of YSOs as compared to the dust pillars shows that while some YSOs are located within dust pillars, many more stars and YSOs reside just outside pillar heads. We suggest that pillars are transient phenomena, part of a continuous outwardly propagating wave of star formation driven by feedback from massive stars. As the pillars are destroyed, they leave newly formed stars in their wake, and these are then subsumed into the young OB association. The YSOs are found predominantly in the cavity between pillars and massive stars, arguing that their formation was in fact triggered. Altogether, the current generation of YSOs shows no strong deviation from a normal initial mass function (IMF). The number of YSOs is consistent with a roughly constant star-formation rate over the past ! 3 Myr, implying that propagating star formation in pillars constitutes an important mechanism to construct unbound OB associations. These accelerated pillars may give birth to massive O-type stars that, after several Myr, could appear to have formed in isolation.

ARECIBO OBSERVATIONS OF FORMALDEHYDE AND RADIO RECOMBINATION LINES TOWARD ULTRACOMPACT H II REGIONS

We report observations of the H110α radio recombination line and H2CO (110-111) toward 21 ultracompact H II regions with the Arecibo 305 m radio telescope. We detect the H110α line in 20 sources, and for each of these we also detect a H2CO absorption feature at nearly the same velocity, demonstrating the association between molecular and ionized gas. We determine kinematic distances and resolve the distance ambiguity for all observed H II regions, as well as for 19 intervening molecular clouds. A plot of the Galactic distribution of these objects traces part of the spiral structure in the first Galactic quadrant. We compare flux densities and velocities as measured with the Arecibo Telescope with interferometric measurements of our sample of ultracompact H II regions. In general, the single-dish fluxes exceed the interferometric values, consistent with an extended component of radio continuum emission.

Ultracompact HII regions: the impact of newly formed massive stars on their environment

SummaryUltracompact (UC)HII regions are manifestations of newly formed massive stars that are still embedded in their natal molecular cloud. They are among the brightest and most luminous single objects in the Galaxy at far infrared and radio wavelengths. Recent high spatial resolution studies, particularly at radio wavelengths, have greatly contributed to our understanding of these dynamic objects and the impact they have on their environment. A summary is given of our current understanding of the physical properties, morphologies, dynamics, number and distribution in the Galaxy, and molecular environments of UCHII regions. Recent models of the circumnebular dust imply that the graphite/silicate abundance ratio is about half that of dust in the diffuse interstellar medium. The dust cocoons are large, cool, and optically thick shortward of a few microns. There are apparently between 1700 and 3000 UCHII regions in the Galaxy. This represents 10–20% of the total O star population. There are too many UCHII regions (just counting those studied with the VLA) to be consistent with the short dynamical lifetimes of this very compact stage of evolution. Both the morphologies and the large number can be understood if UC HII regions are bow shocks. Models of stellar wind supported bow shocks are discussed and consequences for the dynamics and morphologies of the ionized and molecular gas are explored.

DUSTY WIND-BLOWN BUBBLES

Spurred by recent observations of 24 ?m emission within wind-blown bubbles (WBBs), we study the role that dust can play in such environments and build an approximate model of a particular WBB, N49. First, we model the observations with a dusty WBB, and then ask whether dust could survive within N49 to its present age (estimated to be 5 ? 105 to 106 yr). We find that dust sputtering and especially dust-gas friction would imply relatively short timescales (t ~ 104 yr) for dust survival in the wind-shocked region of the bubble. To explain the 24 ?m emission, we postulate that the grains are replenished within the WBB by destruction of embedded, dense cloudlets of interstellar medium gas that have been overrun by the expanding WBB. We calculate the ablation timescales for cloudlets within N49 and find approximate parameters for the embedded cloudlets that can replenish the dust; the parameters for the cloudlets are roughly similar to those observed in other nebula. Such dust will have an important effect on the bubble: including simple dust cooling in a WBB model for N49, we find that the luminosity is higher by approximately a factor of 6 at a bubble age of about 104 yr. At ages of 107 yr, the energy contained in the bubble is lower by about a factor of 8 if dust is included; if dust must be replenished within the bubble, the associated accompanying gas mass will also be very important to WBB cooling and evolution. While more detailed models are certainly called for, this work illustrates the possible strong importance of dust in WBBs, and is a first step toward models of dusty WBBs.