Browsing by Author "Singh, H. P."

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    Near-infrared observations of RR Lyrae and Type II Cepheid variables in the metal-rich bulge globular cluster NGC 6441
    (2022) Bhardwaj, A.; Kanbur, S. M.; Rejkuba, M.; Marconi, M.; Catelan, Márcio; Ripepi, V.; Singh, H. P.
    Context. NGC 6441 is a bulge globular cluster (GC) with an unusual horizontal branch morphology and a rich population of RR Lyrae (RRL) and Type II Cepheid (T2C) variables that is unexpected for its relatively high metallicity. Aims: Our goal is to characterize the pulsation properties of the population II RRL and T2C variables in this metal-rich GC and compare them with a sample of variables in more metal-poor clusters, as well as with theoretical predictions. Methods: We present JHKs time-series observations of 42 RRLs, eight T2Cs, and ten eclipsing binary candidate variables in NGC 6441 . The multi-epoch observations were obtained using the FLAMINGOS-2 instrument on the 8-m Gemini South telescope. Multiband data are used to investigate pulsation properties of the RRL and T2C variables, including their light curves, instability strip, period-amplitude diagrams, and period-luminosity and period-Wesenheit relations (PLRs and PWRs) in the JHKs filters. Results: The near-infrared (NIR) pulsation properties of RRL variables are well fitted with theoretical models that have canonical helium content and the mean-metallicity of NGC 6441 ([Fe/H] = −0.44 ± 0.07 dex). The helium-enhanced RRL models predict brighter NIR magnitudes and bluer colors than the observations of RRL in the cluster. We find that the empirical slopes of the RRL PLRs and PWRs in NGC 6441 are statistically consistent with those of RRLs in more metal-poor GCs, as well as the theoretical models. Therefore, we use theoretical calibrations of RRL period-luminosity-metallicity (PLZ) relations to simultaneously estimate the mean reddening, E(J − Ks)=0.26 ± 0.06 mag, and the distance, d = 12.67 ± 0.09 kpc, to NGC 6441. Our mean reddening value is consistent with an independent estimate from the bulge reddening map based on red clump stars. The RRL-based distance agrees well with similar literature measurements based on the PLZ relations, and the Gaia and other independent methods. Our distance and reddening values provide a very good agreement between the PLRs of T2Cs in NGC 6441 and those for RRLs and T2Cs in Galactic GCs that span a broad range of metallicity. Conclusions: We conclude that the NIR colour-magnitude diagrams, pulsation properties, and PLRs for RRLs and T2Cs in NGC 6441 are in good agreement with the predictions of RRL pulsation models with canonical helium content. This suggests that these population II variables are either not significantly helium enhanced, as previously thought, or the impact of such enhancement is smaller in NIR than the predictions of the pulsation models. Full Table 2 is only available at the CDS via anonymous ftp to https://cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/668/A59...
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    The dusty aftermath of a rapid nova: V5579 Sgr
    (2024) Raj, Ashish; Bisht, Mohit Singh; Walter, F. M.; Pandey, R.; Woodward, C. E.; Harker, D. E.; Bisht, Devendra; Singh, H. P.; Agarwal, A.; Pandey, Jeewan; Joshi, Arti; Belwal, K.; Buil, Christian
    V5579 Sgr was a fast nova discovered in 2008 April 18.784 UT. We present the optical spectroscopic observations of the nova observed from the Castanet Tolosan, SMARTS and CTIO observatories spanning over 2008 April 23 to 2015 May 11. The spectra are dominated by hydrogen Balmer, Fe II and O I lines with P-Cygni profiles in the early phase, typical of an Fe II class nova. The spectra show He I and He II lines along with forbidden lines from N, Ar, S, and O in the nebular phase. The nova showed a pronounced dust formation episode that began about 20 days after the outburst. The dust temperature and mass were estimated using the WISE data from spectral energy distribution (SED) fits. The PAH-like features are also seen in the nova ejecta in the mid-IR Gemini spectra taken 522 d after the discovery. Analysis of the light curve indicates values of t2 and t3 about 9 and 13 days, respectively, placing the nova in the category of fast nova. The best fit cloudy model of the early decline phase JHK spectra obtained on 2008 May 3 and the nebular optical spectrum obtained on 2011 June 2 shows a hot white dwarf source with T-BB similar to 2.6 x 10(5) K having a luminosity of 9.8 x 10(36) ergs s(-1). Our abundance analysis shows that the ejecta is significantly enhanced relative to solar, O/H = 32.2, C/H = 15.5 and N/H = 40.0 in the early decline phase and O/H = 5.8, He/H = 1.5 and N/H = 22.0 in the nebular phase.