NGC 3105: A Young Cluster in the Outer Galaxy

Images and spectra of the open cluster NGC 3105 have been obtained with GMOS on Gemini South. The (i', g'-i') color-magnitude diagram (CMD) constructed from these data extends from the brightest cluster members to g'~23. This is 4 - 5 mag fainter than previous CMDs at visible wavelengths and samples cluster members with sub-solar masses. Assuming a half-solar metallicity, comparisons with isochrones yield a distance of 6.6+/-0.3 kpc. An age of at least 32 Myr is found based on the photometric properties of the brightest stars, coupled with the apparent absence of pre-main sequence stars in the lower regions of the CMD. The luminosity function of stars between 50 and 70 arcsec from the cluster center is consistent with a Chabrier lognormal mass function. However, at radii smaller than 50 arcsec there is a higher specific frequency of the most massive main sequence stars than at larger radii. Photometry obtained from archival SPITZER images reveals that some of the brightest stars near NGC 3105 have excess infrared emission, presumably from warm dust envelopes. Halpha emission is detected in a few early-type stars in and around the cluster, building upon previous spectroscopic observations that found Be stars near NGC 3105. The equivalent width of the NaD lines in the spectra of early type stars is consistent with the reddening found from comparisons with isochrones. Stars with i'~18.5 that fall near the cluster main sequence have a spectral-type A5V, and a distance modulus that is consistent with that obtained by comparing isochrones with the CMD is found assuming solar neighborhood intrinsic brightnesses for these stars.Comment: To appear in The Astrophysical Journa

The Open Cluster NGC 2437 (Messier 46)

The stellar content of the open cluster NGC 2437 (Messier 46) is investigated using moderately deep u*,g', and r' MegaCam images. When compared with solar metallicity isochrones, the (g', u'-g') and (r', g'-r') CMDs are consistent with an age log(t_yr) = 8.35 +/- 0.15, a distance modulus 11.05 +/- 0.05, and a color excess E(B-V) = 0.115 +/- 0.035. The r' luminosity function (LF) of main sequence stars in the magnitude range r' 0.8 solar) has a shape that follows solar neighborhood star counts. However, at fainter magnitudes the cluster LF is flat, in contrast with what would be expected from solar neighborhood counts. The clustering properties of stars in NGC 2437 are investigated by examining the two-point angular correlation functions of main sequence stars in different brightness ranges. Main sequence stars fainter than r' = 17 are less centrally concentrated than brighter stars and are found over a larger area of the sky, suggesting that there is a corona of faint main sequence stars around NGC 2437. Based on the flat LF and extended spatial distribution of faint stars, it is concluded that NGC 2437 is actively shedding stars with masses < 0.8 solar.Comment: To appear in the PAS

Haffner 16 Redux: Revisiting a Young Cluster in the Outer Galaxy

Images and spectra recorded with the Gemini Multi-Object Spectrograph on Gemini South are used to investigate the stellar content of the open cluster Haffner 16. The (i', g'-i') color-magnitude diagram (CMD) constructed from these data samples the cluster main sequence (MS) and 5 magnitudes of the pre-MS (PMS). The isochrones do not track the PMS on the CMD, in the sense that the PMS has a shallower slope on the CMD than predicted by the models. Still, a dip in star counts that is associated with the relaxation of PMS stars onto the MS is identified near i' = 17. The depth and brightness of this feature - as well as the morphology of the cluster MS on the CMD - are matched by models with a slightly sub-solar metallicity that have an age ~ 20 Myr and a distance modulus of 12.3 +/- 0.2. A light profile of Haffner 16 is constructed in the WISE W1 filter which suggests that the cluster is surrounded by a diffuse stellar halo. Spectra are presented of candidate cluster MS and PMS stars selected according to location on the CMD. The spectra show characteristics that are suggestive of a sub-solar metallicity. Halpha emission is common among objects on the PMS locus on the CMD near i' = 18. It is suggested that the location of the Haffner 16 PMS on the CMD is affected by large-scale cool spot activity, likely induced by rapid stellar rotation.Comment: 30 pages total, with 23 figures. The format differs slightly from the published articl

The Distribution of Main Sequence and Pre-Main Sequence Stars in the Young Anticenter Cluster NGC 2401

Images obtained with the Gemini Multi-Object Spectrograph on Gemini South are used to examine the photometric properties and spatial distributions of main sequence (MS) and pre-main sequence (PMS) objects in the star cluster NGC 2401. The data sample several magnitudes fainter than previous studies, and a large population of candidate PMS (cPMS) stars are identified. The cPMS stars are traced out to 2.4 arcmin from the cluster center, and have a flatter spatial distribution than the brightest MS stars near the cluster center. The luminosity function of all MS and candidate PMS stars can be matched by a model that assumes a solar neighborhood mass function, suggesting that NGC 2401 has not yet shed significant numbers of members with masses in excess of 0.5 solar. The frequency of wide binaries among the MS stars is ~3 times higher than among the cPMS stars. It is argued that the difference in the spatial distributions of MS and PMS objects is not the consequence of secular dynamical evolution or structural evolution driven by near-catastrophic mass loss. Rather, it is suggested that the different spatial distributions of these objects is the fossil imprint of primordial sub-clustering that arises naturally if massive stars form preferentially in the highest density central regions of a protocluster.Comment: To appear in the PAS

NGC 5253 and ESO 269-G058: Dwarf Galaxies with a Past

Deep r' and i' images obtained with GMOS on Gemini South are used to probe the bright stellar content in the outer regions of NGC 5253 and ESO269-G058. Red giant branch (RGB) stars are traced out to a distance of 8 kpc along the major axis of NGC 5253, and 6 kpc in ESO269-G058. The outer regions of both galaxies are metal-poor; RGB stars located between projected major axis distances of 2 and 4 kpc in NGC 5253 have [Fe/H] ~ -1, whereas RGB stars in the corresponding portion of ESO269-G058 have [Fe/H] ~ -1.8. Stars with metallicities that differ from the mean by more than a few tenths of a dex make only a modest contribution to the stellar content in the outer regions of both galaxies. A population of bright asymptotic giant branch (AGB) stars is seen in both galaxies. Roughly 1 - 10% of the stellar mass of NGC 5253 may have formed during the past few hundred million years, and it is suggested that the progenitors of the two recent SN Ia in this galaxy may have formed at this time. It is argued that the current episodes of star formation in NGC 5253 and ESO269-G058 may have been triggered up to ~ 1 Gyr in the past. Finally, a distance modulus is computed for each galaxy based on the brightness of the RGB-tip, and the results are 27.48 +/- 0.14 for NGC 5253, and 27.93 +/- 0.18 for ESO269-G058.Comment: 36 pages, including 11 postscript pages. Astronomical Journal in pres