Evidence for Triggered Star Formation in the Horsehead Nebula An Honors Thesis for the Department of Physics and Astronomy Brendan Peter Bowler Tufts University, 2007 Thesis Committee Members: William H. Waller Brian M. Patten William P. Oliver 05/11/2007 Abstract The formation of stars in molecular clouds is subject to both internal and external influences. Triggered star formation inherently involves an exter- nal agent— usually one or more massive stars whose radiation, winds, and eventual supernova(e) can have transformative effects on neighboring clouds. Examples of this triggering process are important for constraining models of induced star formation. The Horsehead Nebula (B33) is a nearby (400 pc) “pillar” of dense gas and dust protruding from the dark cloud L1630 and is seen in silhouette against the bright HII region IC434. I aim to characterize the state and extent of star-formation in this region in order to study the interaction between the massive O9.5V star σ Orionis and this nearby pillar. I present deep IRSF/SIRIUS JHKS and Spitzer/IRAC 4-channel imagery of the Horsehead Nebula. IR color-color and color-magnitude diagrams are used to identify young stellar objects (YSOs) based on their IR excesses and positions indicating youth. When possible, spectral energy distributions and spectral indices of bona fide and candidate YSOs are used to determine IR evolutionary classes. Four young stars are identified in this region. Two flat spectrum protostars are located at the surface of the western limb of the pillar. Two probable transition disk/Class III sources are detected near the base of the pillar and are probably not directly associated with B33. The evolutionary ages of the protostars are similar to the formation timescale of B33 (∼0.2 Myr), suggesting a common formation mechanism as the dense pillar first developed. The radiation-driven implosion model is the likely triggered formation mechanism of the two protostars, as this model satisfies the observed ages, locations, and luminosities of the YSOs in this irradiated cloud. Contents 1 Introduction 4 2 The Horsehead Nebula 11 2.1 Previous Work . 13 3 Observations 17 3.1 InfraRed Survey Facility Near-Infrared Observations . 17 3.1.1 The IRSF and SIRIUS . 17 3.1.2 Data Reduction and Analysis . 18 3.2 SST and Mid-IR Observations . 26 3.2.1 IRAC Observations, Data Reduction, and Analysis . 26 4 Results 28 4.1 Rejection of Extragalactic Contaminants . 28 4.2 Identification of Young Stellar Objects . 31 4.3 Color-Color Diagrams . 32 4.3.1 Near-IR Color-Color Diagram . 32 4.3.2 Mid-IR Color-Color Diagrams . 36 4.3.3 IRSF and IRAC Combined Color-Color Diagrams . 39 4.3.4 Summary of Color-Color Diagrams . 42 4.4 Color-Magnitude Diagrams . 45 4.4.1 Near-IR Color-Magnitude Diagrams . 45 4.4.2 Mid-IR Color-Magnitude Diagram . 51 4.4.3 Summary of Color-Magnitude Diagrams . 51 4.5 Spectral Energy Distributions . 53 1 5 Discussion and Conclusion 61 5.1 The Horsehead as a Site of Triggered Star Formation . 61 5.2 Notes on Individual Sources . 64 5.3 Future Work . 66 2 List of Figures 1.1 NGC 602 in the SMC . 7 2.1 The Horsehead Nebula . 12 3.1 Linear Magnitude Correlation . 24 3.2 Magnitude Transformation Fits . 25 4.1 Rejection of Extragalactic Objects: Mid-IR CMD . 29 4.2 Rejection of Extragalactic Objects: Mid-IR CCDs . 30 4.3 The Near-IR Color-Color Diagram . 33 4.4 Horsehead Near-IR Color-Color Diagram . 35 4.5 IRAC Color-Color Diagrams . 38 4.6 IRSF and IRAC Color-Color Diagrams: A . 41 4.7 IRSF and IRAC Color-Color Diagrams: B . 43 4.8 Near-IR CMD: J vs J − KS ................... 49 4.9 Near-IR CMD: KS vs. H − KS . 50 4.10 Mid-IR CMD: (3.6) vs. (3.6)-(4.5) . 52 4.11 YSO Spectral Energy Distributions . 57 4.12 Uncertain Member Spectral Energy Distributions . 58 4.13 Sources in B33 . 60 3 Chapter 1 Introduction The formation and early evolution of stars and stellar clusters is currently one of the most active fields in astrophysics. Observational and theoretical research primarily over the last thirty years has established a general theory of star formation that covers nearly all stages of a star’s pre-main sequence (PMS) life. However, while the broad notions of the collapse and early evolu- tionary phases of stars of varying masses are thought to be understood, most of the details remain obscure. For example, the fragmentation and accretion of dense cores in molecular clouds is still being investigated. The shape of the initial mass function, the physics of outflows from young stars in the form of Herbig-Haro objects, and disk dissipation mechanisms continue to remain somewhat enigmatic; grain growth and planetesimal formation are also just beginning to be observationally constrained. These topics are but a small portion of the unanswered questions relating to the local star-forming properties of our Milky Way galaxy. There remain untold numbers of related topics in extragalactic star formation studies. One subject that has long been speculated on but has only recently gained the serious attention of researchers involves the interaction between massive young stars and the nearby parental giant molecular clouds from which they were born. There is evidence for mass segregation in young open clusters that cannot be accounted for through dynamical interactions (see Clarke et al. 2000 and Lada & Lada 2003 for brief reviews). Rather than have mi- grated to the centers of clusters, it is thought that massive stars are born in the densest, centralized cores of molecular clouds and remain there through- out their relatively brief lives (a few Myr for O stars). It has also been shown that some young clusters are not in fact coeval, meaning their members were 4 not all formed at the same time. Instead there appears to be an age gra- dient across some clusters, with the younger members generally distributed in the clusters’ outskirts and older members located near the centers (Lee & Chen 2007; Lee et al. 2005; Chen, Lee, & Sanchawala). This age spread is probably dependent on the initial size and density distribution of the origi- nal molecular cloud. However, it is still not clear whether the formation of young clusters occurs in a bimodal fashion (with massive stars formed first and lower-mass stars following) or in a more complicated framework. For example, Hern´andezet al. (2007) find a higher circumstellar disk fraction toward the center of the young 3 Myr cluster σ Orionis. The birth of massive young stars and the formation of open clusters with hundreds to thousands of members is the subject of much current observational and theoretical re- search. Recently Charles and Elizabeth Lada performed a statistical study on nearby (≤ 2 kpc) embedded clusters in molecular clouds (Lada & Lada 2003). While it has been known for decades that stars form in molecular clouds, this study quantitatively described the properties of these newly-formed clusters. They found that most stars are born in embedded clusters of 100 or more members, but that less than 7% of these clusters survive without complete dispersal to an age of 100 Myrs. These remaining clusters have total masses that are at least 50M and are dominated by low-mass members with masses between 0.1–0.6M . This peak of low-mass stars is in good agreement with other observationally derived initial mass function (IMF) studies in star form- ing regions, although the IMF turnover appears to be more closely centered in the 0.1M region (see Bonnell, Larson, & Zinnecker 2007 for a review of recent IMF results). There is also evidence for mass segregation in these nearby embedded clusters, with the most massive stars having formed in the clusters’ centers. Generally by 5 Myrs young clusters are no longer associated with the molecular clouds from which they were born (Lada & Lada 2003). A scenario that partly accounts for this cluster formation and its age spread (with older stars in the centers of clusters) involves a process of in- duced star formation caused by ionizing shock fronts at the surfaces of these eroding molecular clouds. Although this does not explain the results by Hern´andezet al. (2007) regarding the disk fraction of the σ Orionis cluster being higher in the cluster’s center, it should be noted that projection effects of triggering star formation can still account for some of this distribution. For example, molecular cloud material in our line of sight to the cluster’s center may have undergone induced star formation and would therefore create the 5 appearance of disk-bearing young stars in the cluster’s central regions. Stars with masses between roughly 10–50M emit most of their radiation in the ultraviolet portion of the spectrum. These O and B stars make up OB associations, the younger counterparts to older open clusters. The environ- ments surrounding these massive stars are characterized by ionized atomic hydrogen caused by photoionization and are called HII regions. As young clusters are generally still associated with their parental molecular clouds, these HII regions often extend to bright-rimmed clouds (BRCs) which mark the spatial extent of young clusters. Evidence for triggered star formation is partially based on the identification of young stars in these BRCs. NGC 602 is a young star forming region in the Small Magellanic Cloud that ex- emplifies the scenario of a massive star producing an HII region marked by BRCs at its boundaries (Figure 1.1). These star-forming regions are often characterized by shells, bubbles, and cavities carved out and eroded by the ionization, winds, and supernovae of massive young stars.