The Extended Environment of M17: A Star Formation History

Abstract

M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy. It features a bright H II region erupting as a blister from the side of a giant molecular cloud (GMC). Combining photometry from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) with complementary infrared (IR) surveys, we identify candidate young stellar objects (YSOs) throughout a 1.5 • × 1 • field that includes the M17 complex. The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors. Removing contaminants, we produce a highly-reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex. We fit model spectral energy distributions to these sources and constrain their physical properties. Extrapolating the mass function of 62 intermediate-mass YSOs (M ⋆ > 3 M ⊙ ), we estimate that >1000 stars are in the process of forming in the extended outer regions of M17. The remaining 34 candidate YSOs are found in a 0.17 deg 2 field containing the well-studied M17 H II region and photodissociation region, where bright diffuse mid-IR emission drastically reduces the sensitivity of the GLIMPSE point-source detections.By inspecting IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim of a large shell structure ∼0.5 • in diameter (∼20 pc at 2.1 kpc). We present maps of 12 CO and 13 CO (J = 2 → 1) emission observed with the Heinrich Hertz Telescope. The CO emission shows that the shell is a coherent, kinematic structure associated with M17, centered at v = 19 km s −1 . The shell is an extended bubble outlining the photodissociation region of a faint, diffuse H II region several Myr old. We identify a group of candidate ionizing stars within the bubble. YSOs in our catalog are concentrated around the bubble rim, providing evidence that massive star formation has been triggered by the expansion of the bubble. The formation of the massive cluster ionizing the M17 H II region itself may have been similarly triggered. We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex.

RAPID CIRCUMSTELLAR DISK EVOLUTION AND AN ACCELERATING STAR FORMATION RATE IN THE INFRARED DARK CLOUD M17 SWex

“…Following PW10 and Povich et al (2011), we perform aperture photometry on the MIPSGAL 24 μm mosaics only for IR excess sources identified at shorter wavelengths (see Appendix A.1). This means we do not identify sources that show excess emission only at 24 μm, for example transitional disks, because the high potential for source confusion in the inner Galactic plane, coupled with the lower,~ 5 resolution of MIPS compared to IRAC and 2MASS, generates significant numbers of false matches between 24 μm sources and field stars (Povich et al 2009). …”

“…Following the approach used in numerous previous studies (e.g., Shepherd et al 2007;Whitney et al 2008;Povich et al 2009Povich et al , 2011; PW10) we use our SED fitting results to model the YSO mass function (YMF) for the IR excess pointsource population. In this work we introduce a new set of "naked" stellar models that we use to produce the analogous diskless mass function (DlMF) for the X-ray-selected, non-IR excess population lacking inner dust disks (Appendix B).…”

“…We follow our standard procedure for measuring the number of stars (above the hydrogen-burning limit at M 0.1 M ☉ ) in a model stellar population N pop and their total mass M pop (Shepherd et al 2007;Whitney et al 2008;Povich et al 2009Povich et al , 2011PW10). We assume that the actual mass function follows the Salpeter-Kroupa IMF scaled to match the peak of the observed mass function (Figure 6).…”

“…The IRDC complex is contained within a larger giant molecular cloud (GMC) with a mass of at least2 10 5 M ☉ (Elmegreen et al 1979). In a classic paper proposing a theory for sequential massive star formation triggered by propagating ionization fronts, Elmegreen & Lada (1977) proposed that M17 SWex represents the next generation of massive star formation in the M17 complex, which has already produced dozens of OB stars found in the massive NGC 6618 cluster ionizing the M17 H II region (Hanson et al 1997;Hoffmeister et al 2008) and in the optically revealed OB association to the northeast (Ogura & Ishida 1976;Povich et al 2009). …”

“…For the G014.225-00.506 IRDC-the focus of this work-no maser parallaxes are yet available, so we assume = d 2.0 kpc, the same distance as the M17 H II region. This choice is motivated by the observation that both the IRDC and the M17 GMC are centered at the same = v 20 LSR km s −1 (Povich et al 2009;B13), while the G014.33-00.64 maser has multiple velocity components at 19, 22, and 26 km s −1 (Sato et al 2010). The components at = v 26 LSR km s −1 are by far the strongest emitters, and this is also the velocity peak of the surrounding, lower-density molecular cloud traced by CO ( =  J 3 2) emission observed using the James Clerk Maxwell telescope, part of the highest-resolution CO map obtained to date for the entire M17 complex (C. Beaumont 2012, private communication).…”