TheSpitzerSurvey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the HiiRegion NGC 346

Abstract

We use Spitzer Space Telescope observations from the Spitzer Survey of the Small Magellanic Cloud (S 3 MC) to study the young stellar content of N66, the largest and brightest H ii region in the SMC. In addition to large numbers of normal stars, we detect a significant population of bright, red infrared sources that we identify as likely to be young stellar objects ( YSOs). We use spectral energy distribution (SED) fits to classify objects as ordinary (main-sequence or red giant) stars, asymptotic giant branch stars, background galaxies, and YSOs. This represents the first large-scale attempt at blind source classification based on Spitzer SEDs in another galaxy. We firmly identify at least 61 YSOs, with another 50 probable YSOs; only one embedded protostar in the SMC was reported in the literature prior to the S 3 MC. We present color selection criteria that can be used to identify a relatively clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the infrared-bright YSOs in N66 have stellar masses ranging from 2 to 17 M , and that approximately half of the objects are stage II protostars, with the remaining YSOs roughly evenly divided between stage I and stage III sources. We find evidence for primordial mass segregation in the H ii region, with the most massive YSOs being preferentially closer to the center than lower mass objects. Despite the low metallicity and dust content of the SMC, the observable properties of the YSOs appear consistent with those in the Milky Way. Although the YSOs are heavily concentrated within the optically bright central region of N66, there is ongoing star formation throughout the complex, and we place a lower limit on the star formation rate of 3:2 ; 10 À3 M yr À1 over the last $1 Myr.

Star formation rates from young-star counts and the structure of the ISM across the NGC 346/N66 complex in the SMC★

“…This value is consistent with the results of Simon et al (2007) who find a SFR >3.2·10 −3 M yr −1 over the last 1 Myr using Spitzer identified YSO candidates.…”

“…An additional argument against the importance of efficient gas removal leading to a cavity in the central part of our map comes from the Spitzer detected embedded YSO candidates (Simon et al 2007;Sewiło et al 2013). These sources are young ( < ∼ 1 Myr) and several of them are detected towards the central cluster.…”

Radio and infrared study of the star-forming region IRAS 20286+4105

“…We use IRAC photometric data for this study. MIR colours are efficient tools to identify young stars, especially to distinguish between stars with disks and envelopes (Allen et al 2004;Megeath et al 2004;Simon et al 2007;Gutermuth et al 2008). In comparison with the NIR (e.g.…”

“…(iii) The third scheme is based on the method proposed by Simon et al (2007). They use the The CC diagrams for the various identification schemes are shown in Figure 3.…”

Star formation in the Small Magellanic Cloud: the youngest star clusters

“…This recent stellar population does not appear to be uniformly distributed within the ionized nebula, but is rather organized in many, likely coeval, subclusters (Sabbi et al 2007) that are coincident with clumps of molecular gas previously detected by Rubio et al (2000). Simon et al (2007) noted that almost all subclusters host one or more YSOs.…”