E. P. Mercer

FIRST GLIMPSE RESULTS ON THE STELLAR STRUCTURE OF THE GALAXY

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~30 million mid-infrared sources toward the inner Galaxy, 10° ≤ |l| ≤ 65° and |b| ≤ 1°, was used to determine the distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N = N0(l/l0)K1(l/l0), where l0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l0 = 17°-30° with a best-fit value in the 4.5 μm band of l0 = 24° ± 4°. Modeling the source distribution as an exponential disk yields a radial scale length of H* = 3.9 ± 0.6 kpc. There is a pronounced north-south asymmetry in source counts for |l| 30°, with ~25% more stars in the north. For l = 10°-30°, there is a strong enhancement of stars of m = 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length Rbar = 4.4 ± 0.5 kpc, tilted by = 44° ± 10° to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l = 26°-28°, 315-34°, and 306°-309° are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (mK-m[8.0] > 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both.

RCW 49 at Mid-Infrared Wavelengths: A GLIMPSE from the Spitzer Space Telescope

The luminous, massive star formation region RCW 49, located in the southern Galactic plane, was imaged with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope as part of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) program. The IRAC bands contain polycyclic aromatic hydro- carbon (PAH) features at 3.3, 6.2, 7.7, and 8.6 � m, as well as the Brline. These features are the major contributors to the diffuse emission from RCW 49 in the IRAC bands. The Spitzer IRAC images show that the dust in RCW 49 is distributed in a network of fine filaments, pillars, knots, sharply defined boundaries, bubbles, and bow shocks. The regions immediately surrounding the ionizing star cluster and W-R stars are evacuated of dust by stellar winds and radiation. The IRAC images of RCW 49 suggest that the dust in RCW 49 has been sculpted by the winds and radiation from the embedded luminous stars in the inner 5 0 (inner � 6 pc) of the nebula. At projected angular radii �> 5 0 from the central ionizing cluster, the azimuthally averaged infrared intensity falls off as � � � 3 . Both high-resolution radio and mid-IR images suggest that the nebula is density bounded along its western boundary. The filamentary structure of the dust in RCW 49 suggests that the nebula has a small dust filling factor and, as a consequence, the entire nebula may be slightly density bounded to H-ionizing photons. Subject headingg astrochemistry — dust, extinction — H ii regions — infrared: ISM — ISM: lines and bands

A GLIMPSE OF STAR FORMATION IN THE GIANT H II REGION RCW 49

GLIMPSE imaging using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope indicates that star formation is ongoing in the RCW 9 giant H II region. A photometric comparison of the sources in RCW 49 to a similar area to its north finds that at least 300 stars brighter than 13th magnitude in band [3.6] have infrared excesses inconsistent with reddening due to foreground extinction. These are likely young stellar objects (YSOs) more massive than 2.5 M⊙, suggesting that thousands more low-mass stars are forming in this cloud. Some of the YSOs are massive (B stars) and therefore very young, suggesting that a new generation of star formation is occurring, possibly triggered by stellar winds and shocks generated by the older (2-3 Myr) central massive cluster. The Spitzer IRAC camera has proven to be ideally suited for distinguishing young stars from field stars, and the GLIMPSE survey of the Galactic plane will likely find thousands of new star formation regions.

Identification of Main-Sequence Stars with Mid-Infrared Excesses Using GLIMPSE: β Pictoris Analogs?

SpitzerIRAC3.6–8 � mphotometryobtainedaspartoftheGLIMPSEsurveyhasrevealedmid-infraredexcesses for33fieldstarswithknownspectraltypesina1.2deg 2 fieldcenteredonthesouthernGalacticHiiregionRCW49. These stars comprise a subset of 184 stars with known spectral classification, most of which were preselected to have unusually red IR colors. We propose that the mid-IR excesses are caused by circumstellar dust disks that are either very late remnants of stellar formation or debris disks generated by planet formation. Of these 33 stars, 29 appear to be main-sequence stars on the basis of optical spectral classifications. Five of the 29 main-sequence stars are O or B stars with excesses that can be plausibly explained by thermal bremsstrahlung emission, and four are post–main-sequencestars.TheloneOstarisanO4V((f))ataspectrophotometricdistanceof 3233 þ540 � 535 pcandmay be the earliest member of the Westerlund 2 cluster. Of the remaining 24 main-sequence stars, 18 have spectral energydistributions thatare consistentwithhotdustydebrisdisks,a possible signatureofplanetformation.Modeling the excesses as blackbodies demonstrates that the blackbody components have fractional bolometric disk-to-star luminosity ratios, LIR/L� , ranging from 10 � 3 to 10 � 2 with temperatures ranging from 220 to 820 K. The inferred temperatures are more consistent with asteroid belts than with the cooler temperatures expected for Kuiper belts. Mid-IR excesses are found in all spectral types from late B to early K. Subject headingg circumstellar matter — open clusters and associations: individual (Westerlund 2) — planetary systems: formation

The frequency of mid-infrared excess sources in galactic surveys

We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 μm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSE surveys. Of these, 183 are either OB stars earlier than B8 in which the excess plausibly arises from a thermal bremsstrahlung component or evolved stars in which the excess may be explained by an atmospheric dust component. The remaining 47 stars have spectral classifications B8 or later and appear to be main-sequence or late pre-main-sequence objects harboring circumstellar disks. Six of the 47 stars exhibit multiple signatures characteristic of pre-main-sequence circumstellar disks, including emission lines, near-IR K-band excesses, and X-ray emission. Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth. We modeled the excesses in 26 stars having two or more measurements in excess of the expected photospheres as single-component blackbodies. We determine probable disk temperatures and fractional IR luminosities in the range 191 K < T < 787 K and 3.9 × 10-4 < LIR/L* < 2.7 × 10-1. The majority of our modeled sample (14 stars) have 10-3 < LIR/L* < 10-2 and are consistent with either transition disks or massive debris disks. These objects have fractional IR luminosities and temperatures between those of β Pic-type debris disk systems (LIR/L* ≤ 10-3) and Class II pre-main-sequence systems (LIR/L* 10-1). We estimate a lower limit on the fraction of Tycho-2 Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses to be 1.0% ± 0.3%.

DISCOVERY OF A DISTANT STAR FORMATION REGION USING GLIMPSE

Examination of early, in-orbit checkout (IOC) images of a portion of the Galactic plane obtained by the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope revealed the presence of an extended emission nebula with internal structure. The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) data show this nebula, located at l � 42 � and b � 0N5, contains bright point sources and two nonstellar regions. Ancillary data sets were used to help reveal the nature of this nebula and its exciting objects. In particular, 13 CO J ¼ 1 ! 0 line emission mapped by the Galactic Ring Survey (GRS) shows molecular gas associated with the infrared nebula. The 13 CO radial velocity yields a far-kinematic distance of 11.1 kpc to the nebula, since there is no evidence for H i self-absorption. At 11.1 kpc, the far-infrared luminosity of the nebula is 4:8;10 4 L� , and the mass of its molecular cloud is 1:1;10 4 M� . The spectral energy distribution rises steeply from 2.2 to 100 � m with an absorption feature at 10 � m, exhibiting the shape of a late Class 0 young stellar object (YSO). The radio continuum flux observed toward the nebula is consistent with the free-free emission from one or more massive YSOs (MYSOs) with spectral types in the range O9 to B0. This analysis demonstrates one technique the GLIMPSE team will use for revealing thousands of Galactic star formation regions.