arXiv subjects

G. Sarkar

Publications and source records attributed to G. Sarkar.

6 recordsLinked to original sources

arXiv2024

High-resolution spectroscopy of the variable hot post-AGB star LS 4331 (IRAS 17381-1616)

An analysis of high-resolution ($R\sim48\,000$) optical spectrum of hot (B1Ibe) post-AGB star LS 4331 (IRAS 17381-1616) is presented. The detailed identification of the observed absorption and emission features in the wavelength range 3700-9200 \r{A} is carried out for the first time. From non-LTE analysis of absorption lines the atmospheric parameters and chemical composition of the star are derived. We estimate $T_{\rm eff}=20~900\pm500$ K, $\log g=2.57\pm0.08$, $V_r=-51.7\pm0.8$ km s$^{-1}$, $\xi_{\rm t}=24\pm4$ km s$^{-1}$ and $v \sin i=30\pm5$ km s$^{-1}$. A abundance analysis for C, N, O, Mg, Al, S and Si reveals that the N and O abundance is close to solar while metal underabundances relative to the solar value (i.e. [Mg/H] = -1.04 dex, [Al/H] = -1.20 dex, [Si/H] = -0.46 dex) are found. We conclude that LS 4331 is a high galactic latitude metal-poor and carbon deficient hot post-AGB star. The underabundance of carbon ([C/H]= -0.64 dex) is similar to that found in other hot post-AGB stars and indicates that the star's AGB phase of evolution was terminated before the third dredge up. From the nebular emission lines the plasma diagnostics are derived. The presence of nebular emission lines in the spectrum of LS 4331 indicates that the photoionization of circumstellar envelope has already started. The nebular parameters and expansion velocity of the nebula is derived. Using the Gaia DR3 distance the absolute luminosity of the star is derived and the star's position on the post-AGB evolutionary tracks suggests that its initial main sequence mass is about 1.2$M_{\odot}$. It is also reported that fast irregular brightness variations with an amplitude of up to 0.3 mag in $V$ band have been found in the star, typical of hot post-AGB objects.

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arXiv2020

Analysis of absorption lines in the high resolution spectra of five hot post-AGB candidates

From an analysis of absorption lines in the high resolution spectra we have derived the radial velocities, stellar parameters (Teff, gravity, wind-strength parameter logQ and projected rotational velocity) and abundances (C, N, O, and Si ) of IRAS 17460-3114, IRAS 18131-3008, IRAS 19336-0400, LSE 45 and LSE 163. Abundances are found to be solar, except for a low Si abundance in IRAS 19336-0400 and a mild CNO pattern in LSE 163, that rotates at an unusual large rotational velocity for its spectral classification. Combining the stellar parameters information with Gaia DR2 data we are able to derive absolute magnitudes, radii and luminosities and clarify the possible post-AGB nature of the objects. IRAS 17460-3114 and IRAS 18131-3008 are found to be massive OB stars, whereas IRAS 19336-0400 is found to be a post-AGB star, already showing nebular lines in the spectrum. However, we could not confirm the nature of LSE 45 and LSE 163 as post-AGB stars, although their parameters are much more inconsistent with those of massive stars. In both cases, we find a discrepancy between the spectroscopic mass and that derived from the predictions of post-AGB evolutionary tracks. In addition, LSE 45 lacks nebular lines, that are present in IRAS 19336-0400 at a similar temperature. In the case of LSE 163 the rotational velocity (259+/-15 km/s) would be extremely large for a star evolving to CSPN. The combination of this rotational velocity, the high Galactic latitude, slightly large radial velocity and mild CNO enhancement suggests a history of binary interaction.

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arXiv2019

High-resolution spectroscopy of the high velocity hot post-AGB star IRAS 18379-1707 (LS 5112)

The high-resolution ($R\sim48\,000$) optical spectrum of the B-type supergiant LS 5112, identified as the optical counterpart of the post-AGB candidate IRAS 18379-1707, is analysed. We report the detailed identifications of the observed absorption and emission features in the wavelength range 3700-9200 A for the first time. The absorption line spectrum has been analysed using non-LTE model atmosphere techniques to determine stellar atmospheric parameters and chemical composition. We estimate $T_\text{eff}=18~000\pm1000$ K, $\log g=2.25\pm0.08$, $ξ_{\rm t}=10\pm4$ km/s and $v \sin i=37\pm6$ km/s, and the derived abundances indicate a metal-deficient ([M/H]$\approx-0.6)$ post-AGB star. Chemical abundances of eight different elements were obtained. The estimates of the CNO abundances in IRAS 18379-1707 indicate that these elements are overabundant with [(C+N+O)/S]=+0.5$\pm$0.2 suggesting that the products of helium burning have been brought to the surface as a result of third dredge-up on the AGB. From the absorption lines, we derived heliocentric radial velocity of $V_\text{r}=-124.0\pm0.4$ km/s. We have identified permitted emission lines of O I, N I, Na I, S II, Si II, C II, Mg II and Fe III. The nebula forbidden lines of [N I], [O I], [Fe II], [N II], [S II], [Ni II] and [Cr II] have also been identified. The Balmer lines H$α$, H$β$ and H$γ$ show P-Cygni behaviour clearly indicating post-AGB mass-loss process in the object with the wind velocity up to 170 km/s.

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arXiv2018

Line identification and photometric history of the hot post-AGB star Hen 3-1013 (IRAS 14331-6435)

We present a study of the high-resolution optical spectrum for the hot post-asymptotic giant branch (post-AGB) star, Hen 3-1013, identified as the optical counterpart of the infrared sourceIRAS 14331-6435. For the first time the detailed identifications of the observed absorption and emission features in the wavelength range 3700-9000 Å is carried out. Absorption lines of HI, HeI, CI, NI, OI, NeI, CII, NII, OII, SiII, SII, ArII, FeII, MnII, CrII, TiII, CoII, NiII, SIII, FeIII and SIV were detected. From the absorption lines, we derived heliocentric radial velocities of $V_r=-29.6\pm0.4$ km/s. We have identified emission permitted lines of OI, NI, FeII, MgII, SiII and AlII. The forbidden lines of [NI], [FeII], [CrII] and [NiII] have been identified also. Analysis of [NiII] lines in the gaseous shell gives an estimate for the electron density $N_e\sim10^7$ cm$^{-3}$ and the expansion velocity of the nebula $V_{exp}=12$ km/s. The mean radial velocity as measured from emission features of the envelope is $V_r=-36.0\pm0.4$ km/s. The Balmer lines H$α$, H$β$ and H$γ$ show P Cyg behaviour which indicate ongoing post-AGB mass-loss. Based on ASAS and ASAS-SN data, we have detected rapid photometric variability in Hen 3-1013 with an amplitude up to 0.2 mag in the V band. The star's low-resolution spectrum underwent no significant changes from 1994 to 2012. Based on archival data, we have traced the photometric history of the star over more than 100 years. No significant changes in the star brightness have been found.

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arXiv2011

High resolution spectroscopy of the high velocity hot post-AGB star LS III +52 24 (IRAS 22023+5249)

Aims: To investigate the first high resolution optical spectrum of the B-type star, LS III +52 24, identified as the optical counterpart of the hot post-AGB candidate IRAS 22023+5249 (I22023). Methods: We carried out detailed identifications of the observed absorption and emission features in the high resolution spectrum (4290 - 9015 A) of I22023 obtained with the Utrecht Echelle Spectrograph on the 4.2m William Herschel Telescope. Using Kurucz's WIDTH9 program and the spectrum synthesis code, SYNSPEC, we determined the atmospheric parameters and abundances. The photospheric abundances were derived under the LTE approximation. The NEBULAR package under IRAF was used to estimate the electron temperature (T_e) and the electron density (N_e) from the [N II] and [S II] lines. Results: We estimated T_eff=24000 K, log g=3.0, xi_t=7 kms^{-1}. The derived CNO abundances suggest an evolved star with C/O < 1. P-Cygni profiles of hydrogen and helium indicate ongoing post-AGB mass loss. The presence of [N II] and [S II] lines and the non-detection of [O III] indicate that photoionisation has just started. The derived nebular parameters T_e=7000 K, N_e=1.2X10^{4} cm^{-3} are comparable to those measured in young, compact planetary nebulae (PNe). The nebular expansion velocity was estimated to be 17.5 kms^{-1}. Conclusions: The observed spectral features, large heliocentric radial velocity (-148.31 +/- 0.60 kms^{-1}), atmospheric parameters and chemical composition indicate that I22023, at a distance of 1.95 kpc, is an evolved post-AGB star belonging to the old disk population. The nebular parameters suggest that the central star may be evolving into a compact, young PN, similar to Hen3-1357.

astro-ph↗

arXiv2011

High resolution spectroscopy of the high velocity hot post-AGB star LS III +52 24 (IRAS 22023+5249)

The first high-resolution (R~50,000) optical spectrum of the B-type star, LS III +52 24, identified as the optical counterpart of the hot post-AGB candidate IRAS 22023+5249 (I22023) is presented. We report the detailed identifications of the observed absorption and emission features in the full wavelength range (4290-9015 A) as well as the atmospheric parameters and photospheric abundances (under the Local Thermodinamic Equilibrium approximation) for the first time. The nebular parameters (Te, Ne) are also derived. We estimate Teff=24,000 K, log g=3.0, xi=7 kms-1 and the derived abundances indicate a slightly metal-deficient evolved star with C/O<1. The observed P-Cygni profiles of hydrogen and helium clearly indicate on-going post-AGB mass loss. The presence of [N II] and [S II] lines and the non-detection of [O III] indicate that photoionisation has just started. The observed spectral features, large heliocentric radial velocity, atmospheric parameters, and chemical composition indicate that I22023 is an evolved post-AGB star belonging to the old disk population. The derived nebular parameters (Te=7000 K, Ne=1.2x104 cm-3) also suggest that I22023 may be evolving into a compact, young low-excitation Planetary Nebula. Our optical spectroscopic analysis together with the recent Spitzer detection of double-dust chemistry (the simultaneous presence of carbonaceous molecules and amorphous silicates) in I22023 and other B-type post-AGB candidates may point to a binary system with a dusty disk as the stellar origin common to the hot post-AGB stars with O-rich central stars.

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