arXiv subjects

A. Bhardwaj

Publications and source records attributed to A. Bhardwaj.

At least 37 records · Page 2Linked to original sources

arXiv2020

Atomic carbon, nitrogen, and oxygen forbidden emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

The N$_2$ and CO-rich and water-depleted comet C/2016 R2 (Pan-STARRS) (hereafter `C/2016 R2') is a unique comet for detailed spectroscopic analysis. We aim to explore the associated photochemistry of parent species, which produces different metastable states and forbidden emissions, in this cometary coma of peculiar composition. We re-analyzed the high-resolution spectra of comet C/2016 R2, which were obtained in February 2018, using the UVES spectrograph of the European Southern Observatory (ESO) Very Large Telescope (VLT). Various forbidden atomic emission lines of [CI], [NI], and [OI] were observed in the optical spectrum of this comet when it was at 2.8 au from the Sun. The observed forbidden emission intensity ratios are studied in the framework of a couple-chemistry emission model. The model calculations show that CO$_2$ is the major source of both atomic oxygen green and red-doublet emissions in the coma of C/2016 R2 (while for most comets it is generally H$_2$O), whereas, CO and N$_2$ govern the atomic carbon and nitrogen emissions, respectively. Our modelled oxygen green to red-doublet and carbon to nitrogen emission ratios are higher by a factor {of 3}, when compared to the observations. These discrepancies can be due to uncertainties associated with photon cross sections or unknown production/loss sources. Our modelled oxygen green to red-doublet emission ratio is close to the observations, when we consider an O$_2$ abundance with a production rate of 30\% relative to the CO production rate. The collisional quenching is not a significant loss process for N($^2$D) though its radiative lifetime is significant ($\sim$10 hrs). Hence, the observed [NI] doublet-emission ratio ([NI] 5198/5200) of 1.22, which is smaller than the terrestrial measurement by a factor {1.4}, is mainly due to the characteristic radiative decay of N($^2$D).

astro-ph.EP↗

arXiv2019

Evidence of topological gap opening in the surface state of Bi$_2$Se$_3$ by proximity to a magnetic insulator

Topological insulators are bulk insulators with exotic surface states, protected under time-reversal symmetry, that hold promise in observing many exciting condensed-matter phenomena. In this report, we show that by having a topological insulator (Bi$_2$Se$_3$) in proximity to a magnetic insulator (EuS), a metal-to-insulator transition in the surface state, attributed to opening of an exchange gap, can be observed whose properties are tunable using bottom gate voltage and external magnetic field. Our study provides evidence of gate-controlled enhanced interface magnetism with the signature of half-integer quantum Hall effect when the Fermi level is tuned into the exchange gap. These results pave the way for using magnetic proximity effect in developing topological electronic devices.

cond-mat.mes-hall↗

arXiv2019

Variability of young stellar objects in the star-forming region Pelican Nebula

We observed a field of $16'\times 16'$ in the star-forming region Pelican Nebula (IC 5070) at $BVRI$ wavelengths for 90 nights spread over one year in 2012-2013. More than 250 epochs in $VRI$-bands are used to identify and classify variables up to $V\sim 21$~mag. We present a catalogue of optical time-series photometry with periods, mean-magnitudes and classifications for 95 variable stars including 67 pre-main-sequence variables towards star-forming region IC 5070. The pre-main-sequence variables are further classified as candidate classical T Tauri and weak-line T Tauri stars based on their light curve variations and the locations on the color-color and color-magnitude diagrams using optical and infrared data together with Gaia DR2 astrometry. Classical T Tauri stars display variability amplitudes up to three times the maximum fluctuation in disk-free weak-line T Tauri stars, which show strong periodic variations. Short-term variability is missed in our photometry within single nights. Several classical T Tauri stars display long-lasting ($\geq 10$ days) single or multiple fading and brightening events up to a couple of magnitudes at optical wavelengths. The typical mass and age of the pre-main-sequence variables from the isochrone-fitting and spectral energy distributions are estimated to be $\le 1~M_\odot$ and $\sim 2$ Myr, respectively. We do not find any correlation between the optical amplitudes or periods with the physical parameters (mass and age) of pre-main-sequence stars.

astro-ph.SR↗

arXiv2018

Time-series analyses of Cepheid and RR Lyrae variables in the wide-field variability surveys

We discuss time-series analyses of classical Cepheid and RR Lyrae variables in the Galaxy and the Magellanic Clouds at multiple wavelengths. We adopt the Fourier decomposition method to quantify the structural changes in the light curves of Cepheid and RR Lyrae variables. A quantitative comparison of Cepheid Fourier parameters suggests that the canonical mass-luminosity models that lie towards the red-edge of the instability strip show a greater offset with respect to observations for short-period Cepheids. RR Lyrae models are consistent with observations in most period bins. We use ensemble light curve analysis to predict the physical parameters of observed Cepheid and RR Lyrae variables using machine learning methods. Our preliminary results suggest that the posterior distributions of mass, luminosity, temperature and radius for Cepheids and RR Lyraes can be well-constrained for a given metal abundance, provided a smoother grid of models is adopted in various input physical parameters.

astro-ph.SR↗

arXiv2018

Structure and kinematics of Type II Cepheids in the Galactic bulge based on near-infrared VVV data

Type II Cepheids (T2Cs) are radially pulsating variables that trace old stellar populations and provide distance estimates through their period-luminosity (PL) relation. We trace the structure of old stars in the Bulge using new distance estimates and kinematic properties of T2Cs. We present new NIR photometry of T2Cs in the bulge from the VVV survey. We provide the largest sample (894 stars) of T2Cs with JHKs observations that have accurate periods from the OGLE catalog. Our analysis makes use of the Ks-band time-series to estimate mean magnitudes and distances by means of the PL relation. To constrain the kinematic properties of our targets, we complement our analysis with proper motions (PMs) based on both VVV and Gaia DR2. We derive an empirical Ks-band PL relation that depends on Galactic longitude and latitude: Ks0=(10.66+-0.02)-(2.21+-0.03)*(logP-1.2)-(0.020+-0.003)*l+(0.050+-0.008)*|b| mag; individual extinction corrections are based on a 3D reddening map. Our targets display a centrally concentrated distribution, with solid evidence of ellipsoidal symmetry--similar to the RR Lyrae (RRL) ellipsoid--and a few halo outliers up to >100 kpc. We obtain a distance from the Galactic center of R0=8.46+-0.03(stat.)+-0.11(syst.) kpc. We also find evidence that the bulge T2Cs belong to a kinematically hot population, as the tangential velocity components (sigma vl*=104.2+-3.0 km/s and sigma vb=96.8+-5.5 km/s) agree within 1.2sigma. Moreover, the difference between absolute and relative PM is in good agreement with the PM of Sgr A* from VLBA measures. We conclude that bulge T2Cs display an ellipsoidal distribution and have kinematics similar to RRLs, which are other tracers of the old, low-mass stellar population. T2Cs also provide an estimate of R0 that agrees excellently well with the literature, taking account of the reddening law.

astro-ph.GA↗

arXiv2018

Near-infrared observations of OGLE classical and type II Cepheid variables in the LMC

We present results from the Large Magellanic Cloud Near-infrared Synoptic Survey (LMCNISS) for classical and type II Cepheid variables that were identified by the Optical Gravitational Lensing Experiment (OGLE-III) catalogue. Multiwavelength time-series data for classical Cepheid variables are used to study light-curve structures as a function of period and wavelength. We exploit a sample of $\sim$1400 classical and $\sim$80 type II Cepheid variables to derive Period--Wesenheit relations that combine both optical and near-infrared data. The new Period--Luminosity and Wesenheit relations are used to estimate distances to several Local Group galaxies (using classical Cepheids) and to Galactic globular clusters (using type II Cepheids). By appealing to a statistical framework, we find that fundamental-mode classical Cepheid Period--Luminosity relations are non-linear around 10--18 days at optical and near-IR wavelengths. We also suggest that a non-linear relation provides a better constraint on the Cepheid Period--Luminosity relation in type Ia Supernovae host galaxies, though it has a negligible effect on the systematic uncertainties affecting the local measurement of the Hubble constant.

astro-ph.SR↗

arXiv2018

Multiwavelength light curve analysis of Cepheid variables

We present results from a detailed analysis of theoretical and observed light curves of classical Cepheid variables in the Galaxy and the Magellanic Clouds. Theoretical light curves of Cepheid variables are based on non-linear convective hydrodynamical pulsation models and the observational data are taken from the ongoing wide-field variability surveys. The variation in theoretical and observed light curve parameters as a function of period, wavelength and metallicity is used to constrain the input physics to the pulsation models, such as the mass-luminosity relations obeyed by Cepheid variables. We also account for the variation in the convective efficiency as input to the stellar pulsation models and its impact on the theoretical amplitudes and Period-Luminosity relations for Cepheid variables.

astro-ph.GA↗

arXiv2018

The Role of Opacities in Stellar Pulsation

We examine the role of opacities in stellar pulsation with reference to Cepheids and RR Lyraes, and examine the effect of augmented opacities on the theoretical pulsation light curves in key temperature ranges. The temperature ranges are provided by recent experimental and theoretical work that have suggested that the iron opacities have been considerably underestimated. For Cepheids, we find that the augmented opacities have noticeable effects in certain period ranges (around $\log P \approx 1$) even though there is a degeneracy with mixing length. We also find significant effects in theoretical models of B-star pulsators.

astro-ph.SR↗

arXiv2018

New variable stars in the Galactic Bulge: I. The bright regime

We report the detection of 1143 variable stars towards the Galactic bulge, including 320 previously uncatalogued variables, using time-series photometry extracted from data obtained with the VIMOS imager at the Very Large Telescope. Observations of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field in the Galactic Bulge were taken over 2 years between March and October at a cadence of $\sim$ 4 days, enabling the detection of variables with periods up to $\sim$100 days. Many of these were already known, but we detected a significant number of new variables, including 26 Cepheids, a further 18 Cepheid candidates, and many contact binaries. Here we publish the catalog of the new variables, containing coordinates, mean magnitudes as well as periods and classification; full light curves for these variables are also made available electronically.

astro-ph.SR↗

arXiv2017

Composition of Jupiter irregular satellites sheds light on their origin

Irregular satellites of Jupiter with their highly eccentric, inclined and distant orbits suggest that their capture took place just before the giant planet migration. We aim to improve our understanding of the surface composition of irregular satellites of Jupiter to gain insight into a narrow time window when our Solar System was forming. We observed three Jovian irregular satellites, Himalia, Elara, and Carme, using a medium-resolution 0.8-5.5 micro m spectrograph on the National Aeronautics and Space Administration (NASA) Infrared Telescope Facility (IRTF). Using a linear spectral unmixing model we have constrained the major mineral phases on the surface of these three bodies. Our results confirm that the surface of Himalia, Elara, and Carme are dominated by opaque materials such as those seen in carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of Himalia and Elara confirm that their surface composition is the same and magnetite is the dominant mineral. A comparison of the spectral shape of Himalia with the two large main C-type asteroids, Themis (D 176 km) and Europa (D 352 km), suggests surface composition similar to Europa. The NIR spectrum of Carme exhibits blue slope up to 1.5 microm and is spectrally distinct from those of Himalia and Elara. Our model suggests that it is compositionally similar to amorphous carbon. Himalia and Elara are compositionally similar but differ significantly from Carme. These results support the hypotheses that the Jupiter irregular satellites are captured bodies that were subject to further breakup events and clustered as families based on their similar physical and surface compositions.

astro-ph.EP↗

arXiv2017

Galactic Bulge Population II Cepheids in the VVV Survey: Period-Luminosity Relations and a Distance to the Galactic Center

We present the near-infrared observations of population II Cepheids in the Galactic bulge from VVV survey. We identify 340 population II Cepheids in the Galactic bulge from VVV survey based on their match with OGLE-III Catalogue. The single-epoch $JH$ and multi-epoch $K_s$ observations complement the accurate periods and optical $(VI)$ mean-magnitudes from OGLE. The sample consisting of BL Herculis and W Virginis subtypes is used to derive period-luminosity relations after correcting mean-magnitudes for the extinction. Our $K_s$-band period-luminosity relation, $K_s = -2.189(0.056)~[\log(P) - 1] + 11.187(0.032)$, is consistent with published work for BL Herculis and W Virginis variables in the Large Magellanic Cloud. We present a combined OGLE-III and VVV catalogue with periods, classification, mean magnitudes and extinction for 264 Galactic bulge population II Cepheids having good-quality $K_s$-band light curves. The absolute magnitudes for population II Cepheids and RR Lyraes calibrated using Gaia and Hubble Space Telescope parallaxes, together with calibrated magnitudes for Large Magellanic Cloud population II Cepheids, are used to obtain a distance to the Galactic center, $R_0=8.34\pm0.03{\mathrm{(stat.)}}\pm0.41{\mathrm{(syst.)}}$, which changes by $^{+ 0.05}_{-0.25}$ with different extinction laws. While noting the limitation of small number statistics, we find that the present sample of population II Cepheids in the Galactic bulge shows a nearly spheroidal spatial distribution, similar to metal-poor RR Lyrae variables. We do not find evidence of the inclined bar as traced by the metal-rich red-clump stars. The number density for population II Cepheids is more limited as compared to abundant RR Lyraes but they are bright and exhibit a wide range in period that provides a robust period-luminosity relation for an accurate estimate of the distance to the Galactic center.

astro-ph.SR↗

arXiv2016

The Puzzling Detection of X-rays From Pluto by Chandra

Using Chandra ACIS-S, we have obtained imaging Xray spectrophotometry of the Pluto system in support of the New Horizons flyby on 14 July 2015. 174 ksec of observations were obtained on 4 visits in Feb 2014 to Aug 2015. We measured a net signal of 6.8 counts and a noise level of 1.2 counts in a comoving 11 x 11 pixel box (100 x 100 R_Pluto) in the 0.31 to 0.60 keV passband for a detection at > 99.95 C.L. The Pluto photons do not match the background spectrum, are coincident with a 90% flux aperture comoving with Pluto, and are not sky source confused. The mean 0.31 to 0.60 keV Xray power from Pluto is 200 MW, in the midrange of Xray power levels seen for known solar system emission sources: auroral precipitation, solar Xray scattering, and charge exchange (CXE) between solar wind (SW) ions & atmospheric neutrals. We eliminate auroral effects as a source, as Pluto has no known magnetic field & the New Horizons Alice UV spectrometer detected no airglow from Pluto during the flyby. Nano-scale atmospheric haze particles could lead to enhanced resonant scattering of solar X-rays from Pluto, but the energy signature of the detected photons does not match the solar spectrum and estimates of Plutos scattered Xray emission are > 100 times below the 3.9e-5 cps found in our observations. CXE emission from SW carbon, nitrogen, and oxygen ions can produce the energy signature seen, and the 6e25 neutral gas escape rate from Pluto deduced from New Horizons data can support the 3.0e24 Xray photons/sec emission rate required by our observations. Using the SW proton density and speed measured by the Solar Wind Around Pluto (SWAP) instrument in the vicinity of Pluto at the time of the photon emissions, we find too few SW minor ions flowing into the 11 x 11 pixel box centered on Pluto than are needed to support the observed emission rate unless the SW is significantly focused and enhanced in this region.

astro-ph.EP↗

arXiv2015

Modulation of N- to C-terminal interactions enhances protein stability

Although, several factors have been attributed to thermostability, the stabilization strategies used by proteins are still enigmatic. Studies on recombinant xylanase which has the ubiquitous (\b{eta}/α)8 TIM (Triosephosphate isomerase) barrel fold showed that, just a single extreme N-terminus mutation (V1L) markedly enhanced the thermostability by 5 °C without loss of catalytic activity whereas another mutation, V1A at the same position decreased the stability by 2 °C. Based on computational analysis of their crystal structures including residue interaction network, we established a link between N- to C-terminal contacts and protein stability. We demonstrate that augmenting of N- to C-terminal non-covalent interactions is associated with the enhancement of protein stability. We propose that the strategy of mutations at the termini could be exploited with a view to modulate stability without compromising on enzymatic activity, or in general, protein function, in diverse folds where N- and C-termini are in close proximity. Finally, we discuss the implications of our results for the development of therapeutics involving proteins and for designing effective protein engineering strategies.

q-bio.BM↗

arXiv2014

Forbidden oxygen lines at various nucleocentric distances in comets

To study the formation of the [OI] lines - i.e., 5577 A (the green line), 6300 A and 6364 A (the two red lines) - in the coma of comets and to determine the parent species of the oxygen atoms using the green to red-doublet emission intensity ratio (G/R ratio) and the lines velocity widths. We acquired at the ESO VLT high-resolution spectroscopic observations of comets C/2002 T7 (LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and, 103P/Hartley 2 when they were close to the Earth (< 0.6 au). Using the observed spectra, we determined the intensities and the widths of the three [OI] lines. We have spatially extracted the spectra in order to achieve the best possible resolution of about 1-2", i.e., nucleocentric projected distances of 100 to 400 km depending on the geocentric distance of the comet. We have decontaminated the [OI] green line from C2 lines blends. It is found that the observed G/R ratio on all four comets varies as a function of nucleocentric projected distance. This is mainly due to the collisional quenching of O(1S) and O(1D) by water molecules in the inner coma. The observed green emission line width is about 2.5 km/s and decreases as the distance from the nucleus increases which can be explained by the varying contribution of CO2 to the O(1S) production in the innermost coma. The photodissociation of CO2 molecules seems to produce O(1S) closer to the nucleus while the water molecule forms all the O(1S) and O(1D) atoms beyond 1000 km. Thus we conclude that the main parent species producing O(1S) and O(1D) in the inner coma is not always the same. The observations have been interpreted in the framework of the coupled-chemistry-emission model of Bhardwaj & Raghuram (2012) and the upper limits of CO2 relative abundances are derived from the observed G/R ratios. Measuring the [OI] lines could indeed provide a new way to determine the CO2 relative abundance in comets.

astro-ph.EP↗

arXiv2013

The Hot and Energetic Universe: Solar system and exoplanets

The high resolution non-dispersive spectroscopy and unprecedented sensitivity of Athena+ will revolutionize solar system observing: the origin of the ions producing Jupiter's X-ray aurorae via charge exchange will be conclusively established, as well as their dynamics, giving clues to their acceleration mechanisms. X-ray aurorae on Saturn will be searched for to a depth unattainable by current Earth-bound observatories. The X-ray Integral Field Unit of Athena+ will map Mars' expanding exosphere, which has a line-rich solar wind charge exchange spectrum, under differing solar wind conditions and through the seasons; relating Mars' X-ray emission to its atmospheric loss will have significant impact also on the study of exoplanet atmospheres. Spectral mapping of cometary comae, which are spectacular X-ray sources with extremely line-rich spectra, will probe solar wind composition and speed at varying distances from the Sun. Athena+ will provide unique contributions also to exoplanetary astrophysics. Athena+ will pioneer the study of ingress/eclipse/egress effects during planetary orbits of hot-Jupiters, and will confirm/improve the evidence of Star-Planet Interactions (SPI) in a wider sample of planetary systems. Finally Athena+ will drastically improve the knowledge of the X-ray incident radiation on exoplanets, a key element for understanding the effects of atmospheric mass loss and of the chemical and physical evolution of planet atmospheres, particularly relevant in the case of young systems.

astro-ph.HE↗

arXiv2013

Remote Energetic Neutral Atom Imaging of Electric Potential Over a Lunar Magnetic Anomaly

The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human exploration on the Moon. A large positive electric potential was predicted and detected from single point measurements. Here, we demonstrate a remote imaging technique of electric potential mapping at the lunar surface, making use of a new concept involving hydrogen neutral atoms derived from solar wind. We apply the technique to a lunar magnetized region using an existing dataset of the neutral atom energy spectrometer SARA/CENA on Chandrayaan-1. Electrostatic potential larger than +135 V inside the Gerasimovic anomaly is confirmed. This structure is found spreading all over the magnetized region. The widely spread electric potential can influence the local plasma and dust environment near the magnetic anomaly.

physics.space-ph↗

arXiv2010

Dynamics of solar wind protons reflected by the Moon

Solar system bodies that lack a significant atmosphere and significant internal magnetic fields, such as the Moon and asteroids, have been considered as passive absorbers of the solar wind. However, ion observations near the Moon by the SELENE spacecraft show that a fraction of the impacting solar wind protons are reflected by the surface of the Moon. Using new observations of the velocity spectrum of these reflected protons by the SARA experiment on-board the Chandrayaan-1 spacecraft at the Moon, we show by modeling that the reflection of solar wind protons will affect the global plasma environment. These global perturbations of the ion fluxes and the magnetic fields will depend on microscopic properties of the object's reflecting surface. This solar wind reflection process could explain past ion observations at the Moon, and the process should occur universally at all atmosphereless non-magnetized objects.

physics.space-ph↗

arXiv2009

X-rays from Saturn: A study with XMM-Newton and Chandra over the years 2002-05

We present the results of the two most recent (2005) XMM-Newton observations of Saturn together with the re-analysis of an earlier (2002) observation from the XMM-Newton archive and of three Chandra observations in 2003 and 2004. While the XMM-Newton telescope resolution does not enable us to resolve spatially the contributions of the planet's disk and rings to the X-ray flux, we can estimate their strengths and their evolution over the years from spectral analysis, and compare them with those observed with Chandra. The spectrum of the X-ray emission is well fitted by an optically thin coronal model with an average temperature of 0.5 keV. The addition of a fluorescent oxygen emission line at ~0.53 keV improves the fits significantly. In accordance with earlier reports, we interpret the coronal component as emission from the planetary disk, produced by the scattering of solar X-rays in Saturn's upper atmosphere, and the line as originating from the Saturnian rings. The strength of the disk X-ray emission is seen to decrease over the period 2002 - 2005, following the decay of solar activity towards the current minimum in the solar cycle. By comparing the relative fluxes of the disk X-ray emission and the oxygen line, we suggest that the line strength does not vary over the years in the same fashion as the disk flux. We consider possible alternatives for the origin of the line. The connection between solar activity and the strength of Saturn's disk X-ray emission is investigated and compared with that of Jupiter. We also discuss the apparent lack of X-ray aurorae on Saturn and conclude that they are likely to lie below the sensitivity threshold of current Earth-bound observatories. A similar comparison for Uranus and Neptune leads to the same disappointing conclusion.

astro-ph.EP↗