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HST/WFC3 Observations of Low-mass Globular Clusters AM 4 and Palomar 13: Physical Properties and Implications for Mass Loss

Hamren, Katherine M. and Smith, Graeme H. and Guhathakurta, Puragra and Dolphin, Andrew E. and Weisz, Daniel R. and Rajan, Abhijith and Grillmair, Carl J. (2013) HST/WFC3 Observations of Low-mass Globular Clusters AM 4 and Palomar 13: Physical Properties and Implications for Mass Loss. Astronomical Journal, 146 (5). Art. No. 116. ISSN 0004-6256. doi:10.1088/0004-6256/146/5/116.

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We investigate the loss of low-mass stars in two of the faintest globular clusters known, AM 4 and Palomar 13 (Pal 13), using HST/WFC3 F606W and F814W photometry. To determine the physical properties of each cluster—age, mass, metallicity, extinction, and present day mass function (MF)—we use the maximum likelihood color–magnitude diagram (CMD) fitting program MATCH and the Dartmouth, Padova, and BaSTI stellar evolution models. For AM 4, the Dartmouth models provide the best match to the CMD and yield an age of >13 Gyr, metallicity log Z/Z_☉ = −1.68 ± 0.08, a distance modulus (m − M)V = 17.47 ± 0.03, and reddening AV = 0.19 ± 0.02. For Pal 13 the Dartmouth models give an age of 13.4 ± 0.5 Gyr, log Z/Z_☉ = −1.55 ± 0.06, (m − M)V = 17.17 ± 0.02, and AV = 0.43 ± 0.01. We find that the systematic uncertainties due to choice in assumed stellar model greatly exceed the random uncertainties, highlighting the importance of using multiple stellar models when analyzing stellar populations. Assuming a single-sloped power-law MF, we find that AM 4 and Pal 13 have spectral indices α = +0.68 ± 0.34 and α = −1.67 ± 0.25 (where a Salpeter MF has α = +1.35), respectively. Comparing our derived slopes with literature measurements of cluster integrated magnitude (MV) and MF slope indicates that AM 4 is an outlier. Its MF slope is substantially steeper than clusters of comparable luminosity, while Pal 13 has an MF in line with the general trend. We discuss both primordial and dynamical origins for the unusual MF slope of AM 4 and tentatively favor the dynamical scenario. However, MF slopes of more low luminosity clusters are needed to verify this hypothesis.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Guhathakurta, Puragra0000-0001-8867-4234
Dolphin, Andrew E.0000-0001-8416-4093
Weisz, Daniel R.0000-0002-6442-6030
Grillmair, Carl J.0000-0003-4072-169X
Additional Information:© 2013. The American Astronomical Society. Received 12 June 2013. Accepted 20 August 2013. Published 1 October 2013. We would like to thank Jason Kalirai for his invaluable assistance with photometry and data reduction, and Alis Deason, Claire Dorman, Brad Holden, and Connie Rockosi for useful discussions. We would also like to thank Namita Ravi for her work tracking down mass function slopes. Finally, we thank the anonymous referee for his or her helpful comments. This work was supported by the award STScI GO-11680.01-A. K.H. and P.G. acknowledge support from NSF grant AST-1010039 and NASA grant HST-GO-12055.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:globular clusters: individual (AM 4, Palomar 13)
Issue or Number:5
Record Number:CaltechAUTHORS:20131122-102702806
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Official Citation:HST/WFC3 Observations of Low-mass Globular Clusters AM 4 and Palomar 13: Physical Properties and Implications for Mass Loss Katherine M. Hamren et al. 2013 The Astronomical Journal 146 116
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42650
Deposited By: Kristofer Jolley
Deposited On:22 Nov 2013 23:14
Last Modified:10 Nov 2021 16:26

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