arXiv subjects

Athira Unni

Publications and source records attributed to Athira Unni.

23 records · Page 2Linked to original sources

arXiv2022

Carbon abundance of stars in the LAMOST-Kepler field

The correlation between host star iron abundance and the exoplanet occurrence rate is well-established and arrived at in several studies. Similar correlations may be present for the most abundant elements, such as carbon and oxygen, which also control the dust chemistry of the protoplanetary disk. In this paper, using a large number of stars in the Kepler field observed by the LAMOST survey, it has been possible to estimate the planet occurrence rate with respect to the host star carbon abundance. Carbon abundances are derived using synthetic spectra fit of the CH G-band region in the LAMOST spectra. The carbon abundance trend with metallicity is consistent with the previous studies and follows the Galactic chemical evolution (GCE). Similar to [Fe/H], we find that the [C/H] values are higher among giant planet hosts. The trend between [C/Fe] and [Fe/H] in planet hosts and single stars is similar; however, there is a preference for giant planets around host stars with a sub-solar [C/Fe] ratio and higher [Fe/H]. Higher metallicity and sub-solar [C/Fe] values are found among younger stars as a result of GCE. Hence, based on the current sample, it is difficult to interpret the results as a consequence of GCE or due to planet formation.

astro-ph.EP↗

arXiv2022

Galactic chemical evolution of exoplanet host stars: Are high-mass planetary systems young?

The imprints of stellar nucleosynthesis and chemical evolution of the galaxy can be seen in different stellar populations, with older generation stars showing higher $α$-element abundances while the later generations becoming enriched with iron-peak elements. The evolutionary connections and chemical characteristics of circumstellar disks, stars, and their planetary companions can be inferred by studying the interdependence of planetary and host star properties. Numerous studies in the past have confirmed that high-mass giant planets are commonly found around metal-rich stars, while the stellar hosts of low-mass planets have a wide range of metallicity. In this work, we analyzed the detailed chemical abundances for a sample of $>900$ exoplanet hosting stars drawn from different radial velocity and transit surveys. We correlate the stellar abundance trends for $α$ and iron-peak elements with the planets' mass. We find the planet mass-abundance correlation to be primarily negative for $α$-elements and marginally positive or zero for the iron-peak elements, indicating that stars hosting giant planets are relatively younger. This is further validated by the age of the host stars obtained from isochrone fitting. The later enrichment of protoplanetary material with iron and iron-peak elements is also consistent with the formation of the giant planets via the core accretion process. A higher metal fraction in the protoplanetary disk is conducive to rapid core growth, thus providing a plausible route for the formation of giant planets. This study, therefore, indicates the observed trends in stellar abundances and planet mass are most likely a natural consequence of Galactic chemical evolution.

astro-ph.EP↗

arXiv2022

The TESS-Keck Survey. VIII. Confirmation of a Transiting Giant Planet on an Eccentric 261 day Orbit with the Automated Planet Finder Telescope

We report the discovery of TOI-2180 b, a 2.8 $M_{\rm J}$ giant planet orbiting a slightly evolved G5 host star. This planet transited only once in Cycle 2 of the primary Transiting Exoplanet Survey Satellite (TESS) mission. Citizen scientists identified the 24 hr single-transit event shortly after the data were released, allowing a Doppler monitoring campaign with the Automated Planet Finder telescope at Lick Observatory to begin promptly. The radial velocity observations refined the orbital period of TOI-2180 b to be 260.8$\pm$0.6 days, revealed an orbital eccentricity of 0.368$\pm$0.007, and discovered long-term acceleration from a more distant massive companion. We conducted ground-based photometry from 14 sites spread around the globe in an attempt to detect another transit. Although we did not make a clear transit detection, the nondetections improved the precision of the orbital period. We predict that TESS will likely detect another transit of TOI-2180 b in Sector 48 of its extended mission. We use giant planet structure models to retrieve the bulk heavy-element content of TOI-2180 b. When considered alongside other giant planets with orbital periods over 100 days, we find tentative evidence that the correlation between planet mass and metal enrichment relative to stellar is dependent on orbital properties. Single-transit discoveries like TOI-2180 b highlight the exciting potential of the TESS mission to find planets with long orbital periods and low irradiation fluxes despite the selection biases associated with the transit method.

astro-ph.EP↗

arXiv2021

Optical and NIR spectroscopy of cool CEMP stars to probe the nucleosynthesis in low mass AGB binary system

We present the abundance analyses of 7 Carbon enhanced metal-poor (CEMP) stars to understand the origin of carbon in them. We used high-resolution optical spectra to derive abundances of various elements. We also used low-resolution Near-Infrared (NIR) spectra to derive the abundance of O and 12C/13C from the CO molecular band and compared their values with those derived from high-resolution optical spectra. We identified a good agreement between the values. Thus, in cool CEMP stars, the NIR observations complement the high-resolution optical observations to derive the oxygen abundance and the 12C/13C ratio. This enables us to probe fainter cool CEMP stars using NIR spectroscopy. C, N, O abundances of all the program stars in this study show abundances that are consistent with binary mass transfer from a low-mass low-metallicity Asymptotic Giant Branch (AGB) companion which is further supported by the presence of enhancement in neutron-capture elements and detection of radial velocity variation. One of the stars show abundance patterns similar to a CEMP-s star whereas the abundance pattern of the rest of the stars satisfy the criteria required to classify them as CEMP-r/s stars. The sub-classification of some of the stars studied here is revisited. The abundance of neutron capture elements in these CEMP-r/s stars resembles to that of i-process models where proton ingestion episodes in the companion low-mass low-metallicity AGB stars produce the necessary neutron density required for the onset of i-process.

astro-ph.SR↗

arXiv2021

Activity of the first interstellar comet 2I/Borisov around perihelion: Results from Indian observatories

Comet 2I/Borisov is the first true interstellar comet discovered. Here we present results from observational programs at two Indian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 m telescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at the HCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 31/11/2019 and 22/12/2019, at distances of r$_H$ = 2.013 and 2.031 AU respectively. The comet was inferred to be relatively depleted in Carbon bearing molecules on the basis of low $C_2$ and $C_3$ abundances. We find the production rate ratio, Q($C_2$)/Q(CN) = 0.54 $\pm$ 0.18, pre-perihelion and Q($C_2$)/Q(CN) = 0.34 $\pm$ 0.12 post-perihelion. This classifies the comet as being moderately depleted in carbon chain molecules. Using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface having variation in abundance of carbon chain molecules. From imaging observations we infer a dust-to-gas ratio similar to carbon chain depleted comets of the Solar system. We also compute the nucleus size to be in the range $0.18\leq r \leq 3.1$ Km. Our observations show that 2I/Borisov's behaviour is analogous to that of the Solar system comets.

astro-ph.EP↗