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

• Published in: The Astronomical journal 2017-12, Vol.154 (6), p.233
• Main Authors: Subramaniam, Annapurni, Sahu, Snehalata, Postma, Joseph E., Côté, Patrick, Hutchings, J. B., Darukhanawalla, N., Chung, Chul, Tandon, S. N., Rao, N. Kameswara, George, K., Ghosh, S. K., Girish, V., Mohan, R., Murthy, J., Pati, A. K., Sankarasubramanian, K., Stalin, C. S., Choudhury, S.
• Format: Article
• Language: English
• Subjects: Astronomy; Field of view; Galactic clusters; Globular clusters; globular clusters: individual (NGC 1851); Helium; Hertzsprung-Russell and C-M diagrams; Image acquisition; Metallicity; Photometry; Point spread functions; stars: horizontal-branch; Stellar age; Variable stars; Yttrium
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/aa94c3

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 fitting photometric data derived from images in two far-UV (FUV) filters and one near-UV (NUV) filter 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 Y2 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 Yini 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 field of view, to study the hot stellar population in Galactic globular clusters.