arXiv subjects

Kailash C. Sahu

Publications and source records attributed to Kailash C. Sahu.

At least 37 records · Page 2Linked to original sources

arXiv2014

Microlensing Events by Proxima Centauri in 2014 and 2016: Opportunities for Mass Determination and Possible Planet Detection

We have found that Proxima Centauri, the star closest to our Sun, will pass close to a pair of faint background stars in the next few years. Using Hubble Space Telescope (HST) images obtained in 2012 October, we determine that the passage close to a mag 20 star will occur in 2014 October (impact parameter 1.6"), and to a mag 19.5 star in 2016 February (impact parameter 0.5"). As Proxima passes in front of these stars, the relativistic deflection of light will cause shifts in the positions of the background stars by ~0.5 and 1.5 mas, respectively, readily detectable by HST imaging, and possibly by Gaia and ground-based facilities such as VLT. Measurement of these astrometric shifts offers a unique and direct method to measure the mass of Proxima. Moreover, if Proxima has a planetary system, the planets may be detectable through their additional microlensing signals, although the probability of such detections is small. With astrometric accuracies of 0.03 mas (achievable with HST spatial scanning), centroid shifts caused by Jovian planets are detectable at separations of up to 2.0" (corresponding to 2.6 AU at the distance of Proxima), and centroid shifts by Earth-mass planets are detectable within a small band of 8 mas (corresponding to 0.01 AU) around the source trajectories. Jovian planets within a band of about 28 mas (corresponding to 0.036 AU) around the source trajectories would produce a brightening of the source by >0.01 mag and could hence be detectable. Estimated timescales of the astrometric and photometric microlensing events due to a planet range from a few hours to a few days, and both methods would provide direct measurements of the planetary mass.

astro-ph.EP↗

arXiv2011

The First Detection of Blue Straggler Stars in the Milky Way Bulge

We report the first detections of Blue Straggler Stars (BSS) in the bulge of the Milky Way galaxy. Proper motions from extensive space-based observations along a single sight-line allow us to separate a sufficiently clean and well-characterized bulge sample that we are able to detect a small population of bulge objects in the region of the color-magnitude diagram commonly occupied young objects and blue strgglers. However, variability measurements of these objects clearly establish that a fraction of them are blue stragglers. Out of the 42 objects found in this region of the color-magnitude diagram, we estimate that at least 18 are genuine BSS. We normalize the BSS population by our estimate of the number of horizontal branch stars in the bulge in order to compare the bulge to other stellar systems. The BSS fraction is clearly discrepant from that found in stellar clusters. The blue straggler population of dwarf spheroidals remains a subject of debate; some authors claim an anticorrelation between the normalised blue straggler fraction and integrated light. If this trend is real, then the bulge may extend it by three orders of magnitude in mass. Conversely, we find that the genuinely young (~5Gy or younger) population in the bulge, must be at most 3.4% under the most conservative scenario for the BSS population.

astro-ph.GA↗

arXiv2007

Planets in the Galactic Bulge: Results from the SWEEPS Project

The exoplanets discovered so far have been mostly around relatively nearby and bright stars. As a result, the host stars are mostly (i) in the Galactic disk, (ii) relatively massive, and (iii) relatively metal rich. The aim of the SWEEPS project is to extend our knowledge to stars which (i) are in a different part of the Galaxy, (ii) have lower masses, and (iii) have a large range of metallicities. To achieve this goal, we used the Hubble Space Telescope to search for transiting planets around F, G, K, and M dwarfs in the Galactic bulge. We photometrically monitored 180,000 stars in a dense bulge field continuously for 7 days. We discovered 16 candidate transiting extrasolar planets with periods of 0.6 to 4.2 days, including a new class of ultra-short period planets (USPPs) with P < 1.2 days. Radial-velocity observations of the two brightest candidates support their planetary nature. These results suggest that planets are as abundant in the Galactic bulge as they are in the solar neighborhood, and they are equally abundant around low-mass stars (within a factor 2). The planet frequency increases with metallicity even for the stars in the Galactic bulge. All the USPP hosts are low-mass stars, suggesting either that close-in planets around higher-mass stars are irradiatively evaporated, or that the planets can migrate to close-in orbits only around such old and low-mass stars.

astro-ph↗

arXiv2007

Integral-Field Spectroscopy of the Post Red Supergiant IRC +10420: evidence for an axi-symmetric wind

We present NAOMI/OASIS adaptive-optics assisted integral-field spectroscopy of the transitional massive hypergiant IRC +10420, an extreme mass-losing star apparently in the process of evolving from a Red Supergiant toward the Wolf-Rayet phase. To investigate the present-day mass-loss geometry of the star, we study the appearance of the line-emission from the inner wind as viewed when reflected off the surrounding nebula. We find that, contrary to previous work, there is strong evidence for wind axi-symmetry, based on the equivalent-width and velocity variations of H$α$ and Fe {\sc ii} $λ$6516. We attribute this behaviour to the appearance of the complex line-profiles when viewed from different angles. We also speculate that the Ti {\sc ii} emission originates in the outer nebula in a region analogous to the Strontium Filament of $η$ Carinae, based on the morphology of the line-emission. Finally, we suggest that the present-day axisymmetric wind of IRC +10420, combined with its continued blueward evolution, is evidence that the star is evolving toward the B[e] supergiant phase.

astro-ph↗

arXiv2006

Transiting extrasolar planetary candidates in the Galactic bulge

More than 200 extrasolar planets have been discovered around relatively nearby stars, primarily through the Doppler line shifts owing to the reflex motions of their host stars, and more recently through transits of some planets across the face of the host stars. The detection of planets with the shortest known periods, 1.2 to 2.5 days, has mainly resulted from transit surveys which have generally targeted stars more massive than 0.75 M_sun. Here we report the results from a planetary transit search performed in a rich stellar field towards the Galactic bulge. We discovered 16 candidates with orbital periods between 0.4 and 4.2 days, five of which orbit stars of 0.44 to 0.75 M_sun. In two cases, radial-velocity measurements support the planetary nature of the companions. Five candidates have orbital periods below 1.0 day, constituting a new class of ultra-short-period planets (USPPs), which occur only around stars of less than 0.88 M_sun. This indicates that those orbiting very close to more luminous stars might be evaporatively destroyed, or that jovian planets around lower-mass stars might migrate to smaller radii.

astro-ph↗

arXiv2005

Studying the Galactic Bulge Through Spectroscopy of Microlensed Sources: I. Theoretical Considerations

The observed spectra of the microlensed sources towards the Galactic bulge may be used as a tool for studying the kinematics and extinction effects in the Galactic bulge. In this paper, we first investigate the expected distribution of the microlensed sources as a function of depth within the Galactic bulge. Our analysis takes a magnitude limited microlensing survey into account, and includes the effects of extinction. We show that, in the current magnitude limited surveys, the probability that the source lies at the far side of the bulge is larger than the probability that the source lies at the near side. We then investigate the effects of extinction on the observed spectra of microlensed sources. Kurucz model spectra and the observed extinctions towards the Galactic bulge have been used to demonstrate that the microlensed sources should clearly show the effects of extinction which, in turn, can be used as a statistical measure of the contribution of the disk lenses and bulge lenses at different depths. The spectra of the microlensed sources provide a unique probe to derive the radial velocities of a sample which lies preferentially at the far side of the Galactic bulge. The radial velocities, coupled with the microlensing time scales, can thus be useful in studying the 3-dimensional kinematics of the Galactic bulge.

astro-ph↗

arXiv2003

Microlensing towards the Magellanic Clouds: Nature of the Lenses and Implications for Dark Matter

(Abridged) A close scrutiny of the microlensing results towards the Magellanic clouds reveals that the stars within the Magellanic clouds are major contributors as lenses, and the contribution of MACHOs to dark matter is 0 to 5%. The principal results which lead to this conclusion are the following. (i) Out of the ~17 events detected so far towards the Magellanic Clouds, the lens locations have been determined for four events where the lenses are most likely within the Magellanic clouds. (ii) If caused by MACHOs, the event timescales would imply that the MACHOs in the direction of the LMC and the SMC have masses of the order of 0.5 and 2 solar masses, respectively. This is inconsistent with even the most flattened model of the Galaxy. If caused by stars within the Magellanic Clouds, the masses of the lenses are of the order of 0.2 solar mass for both the LMC as well as the SMC. (iii) The fact that the two observed binary events are caused by lenses within the Magellanic Clouds would suggest that there should be a total of ~20 events caused by lenses within the Magellanic Clouds. This implies that most of the microlensing events observed so far are probably caused by stars within the Magellanic Clouds. (iv) If the microlensing events are caused by MACHOs of 0.5 solar mass, ~15 events should have been detected by now towards the SMC, with timescales of ~40 days. The fact that both the events detected towards the SMC have been shown to be due to self-lensing places severe constraints on the MACHO contribution and suggests that the contribution of MACHOs to dark matter is consistent with zero, with an upper limit of 5%.

astro-ph↗

arXiv2002

Near-Field Microlensing and Its Effects on Stellar Transit Observations by Kepler

In this paper, we explore the astrophysical implications of near-field microlensing and its effects on stellar transit observations, with a special emphasis on the Kepler mission. Kepler is a NASA-approved mission whose goal is to detect a large number of extrasolar, earth-like planets by obtaining near-continuous photometry of > 100,000 F, G, and K dwarfs for four years. The expected photometric precision of Kepler is 90 micromag (achieved in 15 minute samples), at which the effect of microlensing by a transiting companion can be significant. For example, for a solar-type primary transited by a white-dwarf secondary, the maximum depth of the transit is 0.01%, which is almost entirely compensated by the microlensing amplification when the white dwarf is at ~0.05 AU. The combined effect of microlensing and transit increases to a net amplification of 150 micromag at an orbital separation of 0.1 AU, and 2.4 millimag at an orbital separation of 1 AU. Thus, the effect of microlensing can be used to break the degeneracy between a planetary-mass object for which the microlensing effect is negligible, and a more massive object of the same size. For brown dwarfs at orbital separations of a few AU, the effect of microlensing is several percent of the transit depth, and hence the microlensing effect must be taken into account in deriving the physical parameters of the brown dwarf. The microlensing signal caused by a neutron star or a black hole in a binary can be several millimag, far exceeding the transit depth, and potentially detectable even from ground-based observations. Kepler will be sensitive to white dwarfs, neutron stars, and black holes in binaries through their microlensing signatures. These observations can be used to derive the frequency of such compact objects in binaries, and to determine their masses.

astro-ph↗

arXiv2002

Studying the Galactic Bulge Through Spectroscopy of Microlensed Sources: II. Observations

The spectroscopy of microlensed sources towards the Galactic bulge provides a unique opportunity to study (i) the kinematics of the Galactic bulge, particularly its far-side, (ii) the effects of extinction on the microlensed sources, and (iii) the contributions of the bulge and the disk lenses to the microlensing optical depth. We present the results from such a spectroscopic study of 17 microlensed sources carried out using the ESO Faint Object Spectrograph (EFOSC) at the 3.6 m European Southern Observatory (ESO) telescope. The spectra of the unlensed sources and Kurucz model spectra were used as templates to derive the radial velocities and the extinctions of the microlensed sources. It is shown that there is an extinction shift between the microlensed population and the non-microlensed population but there is no apparent correlation between the extinction and the radial velocity. This extinction offset, in our best model, would imply that 65% of the events are caused by self-lensing within the bulge. The sample needs to be increased to about 100 sources to get a clear picture of the kinematics of the bulge.

astro-ph↗

arXiv2002

Determination of Substellar Mass Function of Young Open Clusters Using 2MASS and GSC Data

We present a statistical method to derive the mass functions of open clusters using sky survey data such as the 2 Micron All Sky Survey and the Guide Star Catalogue. We have used this method to derive the mass functions in the stellar/substellar regime of three young, nearby open clusters, namely IC 348, Sigma-Orionis and Pleiades. The mass function in the low mass range (M < 0.5 solar mass) is appreciably flatter than the stellar Salpeter function for all three open clusters. The contribution of objects below 0.5 solar mass to the total mass of the cluster is ~40% and the contribution of objects below 0.08 solar mass to the total is ~4%.

astro-ph↗

arXiv2002

The Spatially-Resolved Mass Function of the Globular Cluster M22

HST imaging of M22 has allowed, for the first time, a detailed and uniform mapping of mass segregation in a globular cluster. Luminosity and mass functions from the turnoff down to the mid to lower main sequence are presented for M22 in annular bins from the centre of the cluster out to five core radii. Within the core, a significant enhancement is seen in the proportion of 0.5-0.8 M(sun) stars compared with their numbers outside the core. Numerical modelling of the spatial mass spectrum of M22 shows that the observed degree of mass segregation can be accounted for by relaxation processes within the cluster. The global cluster mass function for M22 is flatter than the Salpeter IMF and cannot be represented by a single power law.

astro-ph↗

arXiv2002

Substellar Mass Function of Young Open Clusters as Determined through a Statistical Approach Using 2MASS and GSC Data

In this paper we present the mass functions in the substellar regime of three young open clusters, IC 348, $σ$ Orionis and Pleiades, as derived using the data from the 2 Micron All Sky Survey (2MASS) catalogue which has a limiting magnitude of K_s ~ 15, and the latest version of the Guide Star Catalogue (GSC) which has a limiting magnitude of F ~ 21 where F refers to the POSS II IIIa-F passband. Based on recent evolutionary models for low mass stars, we have formulated the selection criteria for stars with masses below 0.5 solar mass. Using a statistical approach to correct for the background contamination, we derive the mass function of objects with masses ranging from 0.5 solar mass down to the substellar domain, well below the Hydrogen Burning Mass Limit. The lowest mass bins in our analysis are 0.025, 0.045 and 0.055 solar masses for IC 348, sigma Orionis and Pleiades, respectively. The resultant slopes of the mass functions are 0.8 +/- 0.2 for IC 348, 1.2 +/- 0.2 for sigma Orionis and 0.5 +/- 0.2 for Pleiades, which are consistent with the previous results. The contribution of objects below 0.5 solar mass to the total mass of the cluster is ~40%, and the contribution of objects below 0.08 solar mass to the total mass is ~4%.

astro-ph↗

arXiv2002

Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalogue

We present refined coordinates and proper motion data for the high proper motion (HPM) stars in the Luyten Half-Second (LHS) catalogue. The positional uncertainty in the original Luyten catalogue is typically >10" and is often >30". We have used the digital scans of the Palomar Observatory Sky Survey (POSS) I and POSS II plates to derive more accurate positions and proper motions of the objects. Out of the 4470 candidates in the LHS catalogue, 4323 objects were manually re-identified in the POSS I and POSS II scans. A small fraction of the stars were not found due to the lack of finder charts and digitized POSS II scans. The uncertainties in the revised positions are typically ~2", but can be as high as ~8" in a few cases; this is a large improvement over the original data. Cross-correlation with the Tycho-2 and Hipparcos catalogues yielded 819 candidates (with m_R < 12). For these brighter sources, the position and proper motion data have been replaced with the more accurate Tycho/Hipparcos data. In total, we have revised proper motion measurements and coordinates for 4040 stars and revised coordinates for 4330 stars, which are presented here.

astro-ph↗

arXiv2001

A Re-examination of the "Planetary" Lensing Events in M22

We have carried out further analysis of the tentative, short-term brightenings reported by Sahu et al. (2001), which were suggested to be possible lensings of Galactic-bulge stars by free-floating planets in the globular cluster M22. Closer examination shows that--unlikely as it may seem--small, point-like cosmic rays had hit very close to the same star in both of a pair of cosmic-ray-split images, which cause the apparent brightenings of stars at the times and locations reported. We show that the observed number of double hits is consistent with the frequency of cosmic rays in WFPC2 images, given the number of stars and epochs observed. Finally, we point to ways in which cosmic rays can be more directly distinguished.

astro-ph↗

arXiv2001

High-velocity white dwarfs: thick disk, not dark matter

We present an alternative interpretation of the nature of the extremely cool, high-velocity white dwarfs identified by Oppenheimer et al (2001) in a high-latitude astrometric survey. We argue that the velocity distribution of the majority of the sample is more consistent with the high-velocity tail of a rotating population, probably the thick disk, rather than a pressure-supported halo system. Indeed, the observed numbers are well matched by predictions based on the kinematics of a complete sample of nearby M dwarfs. Analysing only stars showing retrograde motion gives a local density close to that expected for white dwarfs in the stellar (R^-3.5) halo. Under our interpretation, none of the white dwarfs need be assigned to the dark-matter, heavy halo. However, luminosity functions derived from observations of these stars can set important constraints on the age of the oldest stars in the Galactic Disk.

astro-ph↗

arXiv2000

Discovery of the optical counterpart and early optical observations of GRB990712

We present the discovery observations of the optical counterpart of the gamma-ray burster GRB990712 taken 4.16 hours after the outburst and discuss its light curve observed in the V, R and I bands during the first ~35 days after the outburst. The observed light curves were fitted with a power-law decay for the optical transient (OT), plus an additional component which was treated in two different ways. First, the additional component was assumed to be an underlying galaxy of constant brightness. The resulting slope of the decay is 0.97+/-0.05 and the magnitudes of the underlying galaxy are: V = 22.3 +/- 0.05, R = 21.75 +/- 0.05 and I = 21.35 +/- 0.05. Second, the additional component was assumed to be a galaxy plus an underlying supernova with a time-variable brightness identical to that of GRB980425, appropriately scaled to the redshift of GRB990712. The resulting slope of the decay is similar, but the goodness-of-fit is worse which would imply that either this GRB is not associated with an underlying supernova or the underlying supernova is much fainter than the supernova associated with GRB980425. The galaxy in this case is fainter: V = 22.7 +/- 0.05, R = 22.25 +/- 0.05 and I = 22.15 +/- 0.05; and the OT plus the underlying supernova at a given time is brighter. Measurements of the brightnesses of the OT and the galaxy by late-time HST observation and ground-based observations can thus assess the presence of an underlying supernova.

astro-ph↗

arXiv1999

Astrometric Microlensing of Stars

Due to dramatic improvements in the precision of astrometric measurements, the observation of light centroid shifts in observed stars due to intervening massive compact objects (`astrometric microlensing') will become possible in the near future. Upcoming space missions, such as SIM and GAIA, will provide measurements with an accuracy of 4 to 60 micro-arcsec depending on the magnitude of the observed stars, and an accuracy of about 1 micro-arcsec is expected to be achieved in the more distant future. There are two different ways in which astrometric microlensing signals can be used to infer information: one possibility is to perform astrometric follow-up observations on photometrically detected microlensing events, and the other is to perform a survey based on astrometric observations alone. The probability that the disk stars introduce a centroid shift larger than a threshold at a given time for a given source in the Galactic bulge towards Baade's window reaches 100% for a threshold of 0.7 micro-arcsec, while this probability is 2% for a threshold of 5 micro-arcsec. However, this centroid shift does not vary much during a the timescale of a typical photometric microlensing event. So astrometric follow-ups (e.g. with SIM) are not expected to be disturbed by the statistical astrometric microlensing due to disk stars, so that it is possible to infer additional information about the nature of the lens that caused the photometric event. For the upcoming mission GAIA, we expect ~1000 of the observed stars to show a detectable astrometric microlensing signal within its 5 year lifetime. These events can be used to determine accurate masses of the lenses, and to derive the mass and the scale parameters (length and height) of the Galactic disk.

astro-ph↗

arXiv1999

HST and Palomar Imaging of GRB 990123: Implications for the Nature of Gamma-Ray Bursts and their Hosts

We report on HST and Palomar optical images of the field of GRB 990123, obtained on 8 and 9 February 1999. We find that the optical transient (OT) associated with GRB 990123 is located on an irregular galaxy, with magnitude V=24.20 +/- 0.15. The strong metal absorption lines seen in the spectrum of the OT, along with the low probability of a chance superposition, lead us to conclude that this galaxy is the host of the GRB. The OT is projected within the ~1'' visible stellar field of the host, nearer the edge than the center. We cannot, on this basis, rule out the galactic nucleus as the site of the GRB, since the unusual morphology of the host may be the result of an ongoing galactic merger, but our demonstration that this host galaxy has extremely blue optical to infrared colors more strongly supports an association between GRBs and star formation. We find that the OT magnitude on 1999 Feb 9.05, V = 25.45 +/- 0.15, is about 1.5 mag fainter than expected from extrapolation of the decay rate found in earlier observations. A detailed analysis of the OT light curve suggests that its fading has gone through three distinct phases: an early rapid decline (f_{nu} \propto t^{-1.6} for t < 0.1 days), a slower intermediate decline power-law decay (f_{nu} \propto t^{-1.1} for 0.1 < t < 2 days), and then a more rapid decay (at least as steep as (f_ν \propto t^{-1.8} for t > 2 days). The break to steeper slope at late times may provide evidence that the optical emission from this GRB was highly beamed.

astro-ph↗