The Horizontal Branch Population of NGC 1851 as Revealed by the Ultraviolet Imaging Telescope (UVIT)

Abstract

The Horizontal Branch Population of NGC 1851 as Revealed by the Ultraviolet Imaging Telescope ( UVIT ) Annapurni Subramaniam 1 , Snehalata Sahu 1 , Joseph E. Postma 2 , Patrick Côté 3 , J. B. Hutchings 3 , N. Darukhanawalla 3 , Chul Chung 4 , S. N. Tandon 1 , 5 , N. Kameswara Rao 1 , K. George 1 , S. K. Ghosh 6 , 7 , V. Girish 8 , R. Mohan 1 , J. Murthy 1 , A. K. Pati 1 , K. Sankarasubramanian 1 , 8 , 9 , C. S. Stalin 1 , and S. Choudhury 10 1 Indian Institute of Astrophysics, Koramangala II Block, Bangalore-560034, India; purni@iiap.res.in 2 University of Calgary, 2500 University Drive NW, Calgary, Alberta Canada 3 National Research Council of Canada, Herzberg Astronomy and Astrophysics Program, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada 4 Center for Galaxy Evolution Research, Yonsei University, Seoul 03722, Korea 5 Inter-University Centre for Astronomy and Astrophysics, Pune, India 6 National Centre for Radio Astrophysics, Pune, India 7 Tata Institute of Fundamental Research, Mumbai, India 8 ISRO Satellite Centre, HAL Airport Road, Bangalore 560017, India 9 Center of Excellence in Space Sciences India, Indian Institute of Science Education and Research ( IISER ) , Kolkata, Mohanpur 741246, West Bengal, India 10 Yonsei University Observatory, 120-749, Seoul, Korea Received 2017 September 2; revised 2017 October 5; accepted 2017 October 5; published 2017 November 17 Abstract We present the UV photometry of the globular cluster NGC 1851 using images acquired with the Ultraviolet Imaging Telescope ( UVIT ) onboard the ASTROSAT satellite. Point-spread function fi tting photometric data derived from images in two far-UV ( FUV ) fi lters and one near-UV ( NUV ) fi lter are used to construct color – magnitude diagrams ( CMDs ) , in combination with HST and ground-based optical photometry. In the FUV, we detect only the bluest part of the cluster horizontal branch ( HB ) ; in the NUV, we detect the full extent of the HB, including the red HB, blue HB, and a small number of RR Lyrae stars. UV variability was detected in 18 RR Lyrae stars, and three new variables were also detected in the central region. The UV / optical CMDs are then compared with isochrones of different age and metallicity ( generated using Padova and BaSTI models ) and synthetic HB ( using helium- enhanced Y 2 models ) . We are able to identify two populations among the HB stars, which are found to have either an age range of 10 – 12

Gyr, or a range in Y ini of 0.23 – 0.28, for a metallicity of [ Fe / H ]

=

− 1.2 to − 1.3. These estimations from the UV CMDs are consistent with those from optical studies. The almost-complete sample of the HB stars tends to show a marginal difference in spatial / azimuthal distribution among the blue and red HB stars. Thus, this study showcases the capability of UVIT, with its excellent resolution and large fi eld of view, to study the hot stellar population in Galactic globular clusters.