arXiv subjects

Bacham E. Reddy

Publications and source records attributed to Bacham E. Reddy.

At least 37 records · Page 2Linked to original sources

arXiv2018

Radial velocity comparison of Gaia DR2 and RAVE DR5 survey: a systematic offset in radial velocities among a group of highly accurate radial velocity stars

Here, we report comparative study of radial velocity ($\rm RV$) data of two major surveys: Gaia Data Release 2 and RAVE Data Release 5. We restricted the sample to stars with relatively accurate radial velocities ($σ_{\rm RV_{Gaia}} \leq$ 2 km s$^{-1}$ or $\leq$ 2%, and $σ_{\rm RV_{RAVE}}\leq$ 2 km s$^{-1}$ or $\leq$ 2%). The difference between $\rm RV_{Gaia}$ and $\rm RV_{RAVE}$ for a majority of the sample follows normal distribution with mean = 0.28 km s$^{-1}$ and $σ$ = 1.49 km s$^{-1}$. However, we found a very small group of stars ($\approx 0.08\%$ of the total) for which the difference in radial velocities between the two surveys is significantly larger with an offset of $-$104.50 km s$^{-1}$ with $σ$ = 4.92 km s$^{-1}$. Kinematics based on $\rm RV_{ Gaia }$ suggest that most of the group members belong to the Galactic thin disk which agrees with the group's metallicity range of $-$1.2 to $+$0.5 dex suggesting the offset in radial velocity is probably due to RAVE velocity data for this particular group.

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arXiv2016

Chemical compositions and kinematics of the Hercules stream

An abundance analysis is reported of 58 K giants identified by Famaey et al. (2005) as highly probable members of the Hercules stream selected from stars north of the celestial equator in the Hipparcos catalogue. The giants have compositions spanning the interval [Fe/H] from $-$0.17 to $+$0.42 with a mean value of $+$0.15 and relative elemental abundances [El/Fe] representative of the Galactic thin disc. Selection effects may have biassed the selection from the Hipparcos catalogue against the selection of metal-poor stars. Our reconsideration of the recent extensive survey of FG dwarfs which included metal-poor stars (Bensby et al. 2014) provides a [Fe/H] distribution for the Hercules stream which is similar to that from the 58 giants. It appears that the stream is dominated by metal-rich stars from the thin disc. Suggestions in the literature that the stream includes metal-poor stars from the thick disc are discussed.

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arXiv2012

The Chemical composition of the post-AGB F-supergiant CRL 2688

We present an analysis of a high resolution (R~50,000) optical spectrum of the central region of the proto-planetary nebula CRL 2688. This object is thought to have recently moved off the AGB, and display abundance patterns of CNO and heavy elements that can provide us with important clues to understand the nucleosynthesis, dredge-up and mixing experienced by the envelope of the central star during its AGB stage of evolution. The analysis of the molecular features, presumably originated from the circumstellar matter provides further constraints on the chemistry and velocity of the expanding shell, expelled as a consequence of the strong mass loss experienced by the central star. We confirm that the central star shows a spectrum typical of an F-type supergiant with Teff=7250 K, log g=0.5 and [Fe/H]=-0.3 dex. We find that the abundance pattern of this object is characterized by enhancements of Carbon ([C/Fe]=0.6), Nitrogen ([N/Fe]=1.0) and Na ([Na/Fe]=0.7), similar to other previously known carbon-rich post-AGB stars. Yttrium is also enhanced while the [Ba/Y] ratio is very low (-1.0), indicating that only the light s-process elements are enhanced. The Zinc abundance is found to be normal, [Zn/Fe]=0.0, suggesting that there is no depletion of refractory elements. The Halpha, Na I and K I resonance lines show prominent emission components, whose helio-centric radial velocities are offsetted by -41 km/s relative to the photospheric metal-absorption lines. The molecular features of C_2 and CN also show emission components, whose velocities are consistent with the emission components of the Halpha, Na I, and K I lines. On the other hand, their absorption components are more highly blue shifted than the corresponding emission components, which suggests that the regions where the emission and absorption components arise are expanding at different velocities.

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arXiv2012

Chemical compositions of stars in two stellar streams from the Galactic thick disk

We present abundances for 20 elements for stars in two stellar streams identified by Arifyanto & Fuchs (2006, A&A, 449, 533): 18 stars from the Arcturus stream and 26 from a new stream, which we call AF06 stream, both from the Galactic thick disk. Results show both streams are metal-poor and very old (10$-$14 Gyrs) with kinematics and abundances overlapping with the properties of local field thick disk stars. Both streams exhibit a range in metallicity but with relative elemental abundances that are identical to those of thick disk stars of the same metallicity. These results show that neither stream can result from dissolution of an open cluster. It is highly unlikely that either stream represents tidal debris from an accreted satellite galaxy. Both streams most probably owe their origin to dynamical perturbations within the Galaxy.

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arXiv2011

Origin of lithium enrichment in K giants

In this Letter, we report on a low-resolution spectroscopic survey for Li-rich K giants among 2000 low mass (M <= 3 Msun) giants spanning the luminosity range from below to above the luminosity of the clump. Fifteen new Li-rich giants including four super Li-rich K giants (A(Li) >= 3.2) were discovered. A significant finding is that there is a concentration of Li-rich K giants at the luminosity of the clump or red horizontal branch. This new finding is partly a consequence of the fact that our low-resolution survey is the first large survey to include giants well below and above the RGB bump and clump locations in the HR diagram. Origin of the lithium enrichment may be plausibly attributed to the conversion of 3He via 7Be to 7Li by the Cameron-Fowler mechanism but the location for onset of the conversion is uncertain. Two possible opportunities to effect this conversion are discussed: the bump in the first ascent of the red giant branch (RGB) and the He-core flash at the tip of the RGB. The finite luminosity spread of the Li-rich giants serves to reject the idea that Li enhancement is, in general, a consequence of a giant swallowing a large planet.

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arXiv2010

Homogeneous photospheric parameters and C abundances in G and K nearby stars with and without planets

We present a determination of photospheric parameters and C abundances for a sample of 172 G and K dwarfs, subgiants, and giants with and without detected planets in the solar neighbourhood. The analysis was based on high S/N and high resolution spectra observed with the ELODIE spectrograph, and for which the observational data was publicly available. We intend to contribute precise and homogeneous C abundances in studies that compare the behaviour of light elements in stars, hosting planets or not. This will bring new arguments to the discussion of possible anomalies that have been suggested and will contribute to a better understanding of different planetary formation process. The photospheric parameters were computed through the excitation potential, equivalent widths, and ionisation equilibrium of Fe lines selected in the spectra. C abundances were derived from spectral synthesis applied to prominent molecular head bands of C_2 Swan (5128 and 5165) and to a C atomic line (5380.3). The distribution of [C/Fe] vs. [Fe/H] shows no difference in the behaviour of planet-host stars in comparison with stars for which no planet was detected, for both dwarf and giant subsamples. This result is in agreement with the hypothesis of primordial origin for the chemical abundances presently observed instead of self-enrichment during the planetary system formation and evolution. Additionally, giants are clearly depleted in [C/Fe] (~0.14 dex) when compared with dwarfs, which is probably related to evolution-induced mixing of H-burning products in the envelope of evolved stars. Subgiants, although in small number, seems to follow the same C abundance distribution as dwarfs. We also analysed the kinematics of the sample stars that, in majority, are members of the Galaxy's thin disc. Finally, comparisons with other analogue studies were performed and, within the uncertainties, showed good agreement.

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arXiv2008

Searching for the metal-weak thick disc in the solar neighbourhood

An abundance analysis is presented of 60 metal-poor stars drawn from catalogues of nearby stars provided by Ariyanto et al. (2005) and Schuster et al. (2006). In an attempt to isolate a sample of metal-weak thick disc stars, we applied the kinematic criteria $V_{\rm rot} \geq 100$ km s$^{-1}$, $|U_{LSR}| \leq 140$ km s$^{-1}$, and $|W_{LSR}| \leq 100$ km s$^{-1}$. Fourteen stars satisfying these criteria and having [Fe/H] $\leq -1.0$ are included in the sample of 60 stars. Eight of the 14 have [Fe/H] $\geq -1.3$ and may be simply thick disc stars of slightly lower than average [Fe/H]. The other six have [Fe/H] from -1.3 to -2.3 and are either metal-weak thick disc stars or halo stars with kinematics mimicking those of the thick disc. The sample of 60 stars is completed by eight thick disc stars, 20 stars of a hybrid nature (halo or thick disc stars), and 18 stars with kinematics distinctive of the halo.

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arXiv2006

Abundance analysis of cool extreme helium star: LSS 3378

Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is presented. The abundance analysis is done using LTE line formation and LTE model atmospheres constructed for EHe stars. The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and s-process nucleosynthesis. The derived abundances of iron-peak and alpha-elements indicate absence of selective fractionation or any other processes that can distort chemical composition of these elements. Hence, the Fe abundance (log epsilon(Fe) = 6.1) is adopted as an initial metallicity indicator. The measured abundances of LSS 3378 are compared with those of R Coronae Borealis (RCB) stars and with rest of the EHe stars as a group.

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arXiv2005

Elemental Abundance Survey of The Galactic Thick Disk

[Abridged abstract] We have performed an abundance analysis for 176 F- and G- dwarfs of the Galactic thick disk component. Using accurate radial velocities combined with $Hipparcos$ astrometry, kinematics (U, V, and W) and Galactic orbital parameters were computed. We estimate the probability for a star to belong to the thin disk, the thick disk or the halo. Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, Ce, Nd, and Eu have been obtained. The abundances for thick disk stars are compared with those for thin disk members from Reddy et al. (2003). The ratios of $α$-elements (O, Mg, Si, Ca and Ti) to iron for thick disk disk stars show a clear enhancement compared to thin disk members in the range $-0.3 <$ [Fe/H] $ < -1.2$. There are also other elements -- Al, Sc, V, Co, and possibly Zn -- which show enhanced ratios to iron in the thick disk relative to the thin disk. The abundances of Na, Cr, Mn, Ni, and Cu (relative to Fe) are very similar for thin and thick disk stars. The dispersion in abundance ratios [X/Fe] at given [Fe/H] for thick disk stars is consistent with the expected scatter due to measurement errors, suggesting a lack of `cosmic' scatter. The observed compositions of the thin and thick disks seem to be consistent with models of galaxy formation by hierarchical clustering in a $Λ$CDM universe. In particular, the distinct abundance patterns observed in the thin and thick disks, and the chemical homogeneity of the thick disk at different galactocentric distances favor a scenario in which the majority of thick-disk stars were formed {\it in situ}, from gas rich merging blocks.

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arXiv2005

Abundance Analyses of Field RV Tauri Stars, VI: An Extended Sample

An abundance analysis is presented and discussed for a sample of 14 RV Tauri stars. The present abundance data and those from our previous papers and by other workers are combined in an attempt to further understanding of the dust-gas separation process which afflicts many RV Tauri variables. We propose that a star's intrinsic (i.e., initial) metallicity is given by the photospheric zinc abundance. Variables warmer that about 5000 K and with an initial metallicity [Fe/H] $\geq$ $-$1 are affected by dust-gas separation. Variables of all metallicities and cooler than about $T_{\rm eff} \simeq 5000$ K are unaffected by dust-gas separation. The RV Tauri variables show a spread in their C abundances with the lower boundary of the points in the C versus Zn plane falling close to the predicted trend for giants after the first dredge-up. The upper boundary is inhabited by a few stars that are carbon-rich. The O abundances in the mean follow the predicted trend from unevolved stars in line with the expectation that photospheric O abundance is unaffected by the first dredge-up. An evolutionary scenario involving mass loss by a first ascent or early-AGB red giant, the primary star of a binary, is sketched.

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arXiv2005

Three Li-rich K giants: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952

We report on spectroscopic analyses of three K giants previously suggested to be Li-rich: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952. High-resolution optical spectra and the LTE model atmospheres are used to derive the stellar parameters: ($T_{\rm eff}$, log $g$, [Fe/H]), elemental abundances, and the isotopic ratio $^{12}$C/$^{13}$C. IRAS 13539-4153 shows an extremely high Li abundance of $\logε$(Li) $\approx$ 4.2, a value ten times more than the present Li abundance in the local interstellar medium. This is the third highest Li abundance yet reported for a K giant. IRAS 12327-6523 shows a Li abundances of $\logε$(Li)$\approx$ 1.4. IRAS 17596-3952 is a rapidly rotating ($V{\sin i}$ $\approx$ 35 km s$^{-1}$) K giant with $\logε$(Li) $\approx$ 2.2. Infrared photometry which shows the presence of an IR excess suggesting mass-loss. A comparison is made between these three stars and previously recognized Li-rich giants.

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arXiv2004

High Resolution Spectroscopy of the high galactic latitude RV Tauri star CE Virginis

Analysis of the surface composition of the suspected cool RV Tauri star CE Vir shows no systematic trend in depletions of elements with respect to condensation temperature. However, there is a significant depletion of the elements with respect to the first ionization potential of the element. The derived Li abundance of log $ε$ (Li) = 1.5$\pm$0.2 indicates production of Li in the star. Near infrared colours indicate sporadic dust formation close to the photosphere.

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arXiv2004

High resolution spectroscopy of the hot post-AGB stars : IRAS13266-5551 (CPD-55 5588) and IRAS17311-4924 (Hen3-1428)

The high resolution spectra covering the wavelength range 4900 A to 8250 A of the hot post-AGB stars IRAS13266-5551 (CPD-55 5588) and IRAS17311-4924 (Hen3-1428) reveal absorption lines of CII, NII, OII, AlIII, SiIII and FeIII and a rich emission line spectrum consisting of HI, HeI, CII, NI, OI, MgII, AlII, SiII, VI, MnI, FeIII, [FeII] and [CrII]. The presence of [NII] and [OI] lines and absence of [OIII] indicate low excitation nebulae around these stars. The components of NaI absorption lines indicate the presence of neutral circumstellar envelopes in addition to the low excitation nebulae around these two hot post-AGB stars. The H-alpha lines show P-Cygni profiles indicating ongoing post-AGB mass loss. From the absorption lines, we derived heliocentric radial velocities of 65.31 +/- 0.34 km/s and 27.55 +/- 0.74 km/s for IRAS13266-5551 and IRAS17311-4924 respectively. High galactic latitude and large radial velocity of IRAS13266-5551 indicate that it belongs to the old disk population. Preliminary estimates for the CNO abundances in IRAS13266-5551 are obtained.

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arXiv2004

Lithium Abundances of the Local Thin Disk Stars

Lithium abundances are presented for a sample of 181 nearby F and G dwarfs with accurate {\it Hipparcos} parallaxes. The stars are on circular orbits about the Galactic centre and, hence, are identified as belonging to the thin disk. This sample is combined with two published surveys to provide a catalogue of lithium abundances, metallicities ([Fe/H]), masses, and ages for 451 F-G dwarfs, almost all belonging to the thin disk. The lithium abundances are compared and contrasted with published lithium abundances for F and G stars in local open clusters. The field stars span a larger range in [Fe/H] than the clusters for which [Fe/H] $\simeq 0.0\pm0.2$. The initial (i.e., interstellar) lithium abundance of the solar neighborhood, as derived from stars for which astration of lithium is believed to be unimportant, is traced from $\logε$(Li) = 2.2 at [Fe/H] = -1 to $\logε$(Li) = 3.2 at $+0.1$. This form for the evolution is dependent on the assumption that astration of lit hium is negligible for the stars defining the relation. An argument is advanced that this latter assumption may not be entirely correct, and, the evolution of lithium with [Fe/H] may be flatter than previously supposed. A sharp Hyades-like Li-dip is not seen among the field stars and appears to be replaced by a large spread among lithium abundances of stars more massive than the lower mass limit of the dip. Astration of lithium by stars of masses too low to participate in the Li-dip is discussed. These stars show little to no spread in lithium abundance at a given [Fe/H] and mass.

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arXiv2003

An Abundance Analysis of the New Carbon-Rich Proto-Planetary Nebula IRAS 06530$-$0213

In this paper, we present a study of the proto-planetary nebula (PPN) IRAS 06530$-$0213 based on low- and high-resolution spectra. The low-resolution spectrum shows that star is an F5 supergiant with molecular C$_{2}$ and C$_{3}$ and enhanced s-process lines. From the high-resolution spectra, the following atmospheric parameters were determined: $T_{\rm eff}$ = 6900 K, log$g$ = 1.0 and $ξ_{\rm t}$ = 4.5. Abundance analysis shows that IRAS 06530$-$0213 is metal-poor ([Fe/H]=$-$0.9) and overabundant in carbon ([C/Fe]=1.3), nitrogen ([N/Fe]=1.0), and s-process elements ([s-process/Fe]=1.9), indicating AGB nucleosynthesis and deep convective mixing. From the analysis of circumstellar C$_{2}$ and CN molecular bands in the spectrum of IRAS 06530$-$0213, an envelope expansion velocity of V$_{\rm exp}$=~14$\pm1$ km s$^{-1}$ was determined, a typical value for post-AGB stars. Also typical of PPNs is the double-peaked spectral energy distribution. The properties of both the photosphere and circumstellar envelope suggest that IRAS 06530$-$0213 is unambiguously a low-mass, carbon-rich PPN. For comparison purposes, new, high-resolution spectra of the well-known PPN HD 56126 (IRAS 07134+1005) was also analyzed and compared with previous results.

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arXiv2002

The Chemical Compositions of Galactic Disk F and G Dwarfs

Photospheric abundances are presented for 27 elements from carbon to europium in 181 F-G dwarfs from a differential LTE analysis of high-resolution and high signal-to-noise spectra. Stellar $T_{\rm eff}$ were adopted from an infrared flux method calibration of Strömgren photometry. Stellar log $g$ were calculated from {\it Hipparcos} parallaxes and stellar evolutionary tracks. Stellar space motions ($U, V, W$) and a Galactic potential were used to estimate Galactic orbital parameters. Results of $α$-elements -- O, Mg, Si, Ca, and Ti -- show [$α$/Fe] to increase slightly with decreasing [Fe/H]. Heavy elements with dominant contributions at solar metallicity from the $s$-process show [$s$/Fe] to decrease slightly with decreasing [Fe/H]. Scatter in [X/Fe] at a fixed [Fe/H] is entirely attributable to the small measurement errors, after excluding the few thick disc stars and the $s$-process enriched CH subgiants. Tight limits are set on `cosmic' scatter. By combining our sample with published studies, thick disc stars are identified by their $V_{LSR}$ in the range $- 40$ to -100 km s$^{-1}$. These are very old stars with origins in the inner Galaxy and metallicities [Fe/H] $\leq -0.4$. At the same [Fe/H], the sampled thin disc stars have $V_{LSR} \sim 0$ km s$^{-1}$, and are generally younger with a birthplace at about the Sun's Galactocentric distance. In the range $-$0.35 $\geq$ [Fe/H] $\geq$ $-$0.70, well represented by present thin and thick disc samples, [X/Fe] of the thick disc stars is greater than that of thin disc stars for Mg, Al, Si, Ca, Ti, and Eu. [X/Fe] is very similar for the thin and thick disc for -- notably -- Na, and iron-group elements.

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arXiv2002

A Search for $^{6}$Li in Stars with Planets

Using very high-resolution (R$\sim$ 125,000) and high quality (S/N $\geq$ 350) spectra, we have searched for $^{6}$Li in stars hosting extra-solar planets. From detailed profile-fitting of the Li {\sc i} resonance line at 6707.7 Å, we find no significant amount of $^{6}$Li relative to the $^{7}$Li for any of 8 planet bearing stars ($^{6}$Li/$^{7}$Li $\leq$ 0.0 - 0.03) with a strong Li {\sc i} lines. In particular, we do not confirm the presence of $^{6}$Li with $^{6}$Li/$^{7}$Li = 0.13 reported by Israelian et al.(2001) for HD 82943, a star with two known planets. Several of the 8 stars plus HD 219542 A, the planet-less primary of a binary, have been identified in the literature as possible recipients of accreted terrestrial material. For all of the planet-hosting stars and an additional 5 planet-less stars, we find no $^{6}$Li.

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arXiv2001

Spectroscopic Study of IRAS 19285+0517(PDS 100): A Rapidly Rotating Li-Rich K Giant

We report on photometry and high-resolution spectroscopy for IRAS 19285+0517. The spectral energy distribution based on visible and near-IR photometry and far-IR fluxes shows that the star is surrounded by dust at a temperature of $T_{\rm {d}}$ $\sim$ 250 K. Spectral line analysis shows that the star is a K giant with a projected rotational velocity $v sin i$ = 9 $\pm$ 2 km s$^{-1}$. We determined the atmospheric parameters: $T_{\rm {eff}}$ = 4500 K, log $g$ = 2.5, $ξ_{t}$ = 1.5 km s$^{-1}$, and [Fe/H] = 0.14 dex. The LTE abundance analysis shows that the star is Li-rich (log $ε$(Li) = 2.5$\pm$0.15), but with essentially normal C, N, and O, and metal abundances. Spectral synthesis of molecular CN lines yields the carbon isotopic ratio $^{12}$C/$^{13}$C = 9 $\pm$3, a signature of post-main sequence evolution and dredge-up on the RGB. Analysis of the Li resonance line at 6707 Åfor different ratios $^{6}$Li/$^{7}$Li shows that the Li profile can be fitted best with a predicted profile for pure $^{7}$Li. Far-IR excess, large Li abundance, and rapid rotation suggest that a planet has been swallowed or, perhaps, that an instability in the RGB outer layers triggered a sudden enrichment of Li and caused mass-loss.

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