The Elephant Trunk Nebula and the Trumpler 37 cluster: contribution of triggered star formation to the total population of an H iiregion

Abstract

Rich young stellar clusters produce H II regions whose expansion into the nearby molecular cloud is thought to trigger the formation of new stars. However, the importance of this mode of star formation is uncertain. This investigation seeks to quantify triggered star formation (TSF) in IC 1396A (aka the Elephant Trunk Nebula), a bright-rimmed cloud (BRC) on the periphery of the nearby giant H II region IC 1396 produced by the Trumpler 37 cluster. X-ray selection of young stars from Chandra X-ray Observatory data is combined with existing optical and infrared surveys to give a more complete census of the TSF population. Over 250 young stars in and around IC 1396A are identified; this doubles the previously known population. A spatio-temporal gradient of stars from the IC 1396A cloud towards the primary ionizing star HD 206267 is found. We argue that the TSF mechanism in IC 1396A is a radiation-driven implosion process persisting over several million years. Analysis of the X-ray luminosity and initial mass functions indicates that >140 stars down to 0.1 M were formed by TSF. Considering other BRCs in the IC 1396 H II region, we estimate the TSF contribution for the entire H II region exceeds 14-25 per cent today, and may be higher over the lifetime of the H II region. Such triggering on the periphery of H II regions may be a significant mode of star formation in the Galaxy.

The low-mass stellar population in the young cluster Tr 37

“…Even if we consider that Tr 37 has suffered substantial expansion to arrive to its current stage (at a typical rate of 1 pc/Myr), an initial cluster size of 2 pc would only produce a density of 30 stars/pc 3 . Moreover, Spitzer observations (SA06a) and isochrone considerations (SA05; Getman et al 2012) suggest that the population in the outskirts is most likely younger and formed in-situ, not by expansion of the original cluster. According to Sect.…”

“…Finally, since this survey was fully independent of the X-ray observations of the IC 1396A globule by Getman et al (2012), we also checked the objects in common with X-ray observations. A total of 23 of the X-ray YSO candidates and 16 of the non-Xray YSO with disks were found in common with our member selection, all labeled as sure and probable members.…”

“…This allows us to use the extinction as a further membership criterion, by defining the extinction ranges expected for the cluster members. Since this survey targeted fainter and thus potentially more extincted members, we explored the extinction distribution of our newly found objects with Li I absorption (2000) isochrones for 1, 10, and 100 Myr are displayed, transformed into the Johnsons system according to Fernie (1983) and Getman et al (2012). An extinction vector (Cardelli et al 1989) is also displayed.…”

“…5), we find that their ages are consistent with the mean age of 4 Myr derived previously (SA05). Since the Siess et al isochrones are given in the Cousins system, we transformed them into the Johnsons system following the prescriptions of Fernie (1983, for blue stars, V − I c < 1.5 mag), and Getman et al (2012, for red stars, V − I c > 1.5 mag). A few of the objects appear below the 10 Myr isochrone, but in most cases they are stars with flared disks and high (or even anomalous) extinction being thus probably UXor candidates (all labeled in Table A.1).…”

“…Multiwavelength studies (Sicilia-Aguilar et al 2004, 2006a; from now on SA04, SA05, SA06a, and SA06b) have targeted the solar-type population in the region, finding evidence of substantial disk evolution and dust processing (SA06a; Sicilia-Aguilar et al 2007, 2011b, from now on SA11). Detailed Hα photometry surveys (Barentsen et al 2011) revealed an extended population of accreting stars, some of which could be younger than the main cluster (SA05, Getman et al 2012).…”