arXiv subjects

N. M. Ashok

Publications and source records attributed to N. M. Ashok.

At least 37 records · Page 2Linked to original sources

arXiv2015

IR study of nova V2468 Cyg from early decline to the coronal phase

We present infrared spectroscopic and photometric observations of the nova V2468 Cyg covering the period from 2008 March 13 till 2008 November 11. The JHK spectra of the object have been taken from the Mount Abu Infrared Observatory using the Near-Infrared Imager/Spectrometer. Spectra from 0.8-5.2 micron are also presented that were obtained using the NASA Infrared Telescope Facility and the SPEX instrument. The spectra are dominated by strong H I lines from the Brackett and Paschen series, Fe II, OI and CI lines in the initial days, typical of an Fe II type nova. The lines were broader in the period immediately after outburst with measured FWHM of 1800-2300 km/s for the Pa-beta and Br-gamma lines. These values narrowed to 1500-1600 km/s by 12 days from outburst. The spectra showed prominent He I lines at 1.0830 and 2.0581 micron together with H I and O I emission features after 36 days from outburst. Our IR observations show the comparatively broad emission lines, the rapid development of the spectrum to higher ionization, the early appearance of coronal lines, and the absence of dust emission, all features that indicate the hybrid nature of the nova. This is perhaps the most extensively observed example of a probable Fe IIb type nova at the infrared wavelengths. We also notice a short lived emission line of Fe II at 2.0888 micron which was present between April 9, 2008 to May 9, 2008. No dust emission is seen from the nova ejecta. We have also estimated the range for the ejecta mass in V2468 Cyg to be 3 x 10^{-6} - 10^{-5} Msun.

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arXiv2014

Near-infrared studies of nova V5584 Sgr in the pre-maximum and early decline phase

We present near-infrared spectroscopic and photometric observations of nova V5584 Sgr taken during the first 12 days following its discovery on Oct. 26.439 UT 2009. The evolution of the spectra is shown from the initial P Cygni phase to an emission line phase. The prominent carbon lines seen in the JHK spectra closely match those observed in a FeII class nova outburst. The spectra show first-overtone CO bands in emission between 2.29-2.40 micron. By examining WISE and other publicly available data, we show that the nova underwent a pronounced dust formation phase during February - April 2010.

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arXiv2014

The hybrid, coronal lines nova V5588 Sgr (2011 N.2) and its six repeating secondary maxima

The outburst of Nova Sgr 2011 N.2 (=V5588 Sgr) was followed with optical and near-IR photometric and spectroscopic observations for 3.5 years, beginning shortly before the maximum. V5588 Sgr is located close to Galactic center, suffering from E(B-V)=1.56 (+/-0.1) extinction. The primary maximum was reached at V=12.37 on UT 2011 April 2.5 (+/-0.2), and the underlying smooth decline was moderately fast with t(2,V)=38 and t(3,V)=77 days. On top of an otherwise normal decline, six self-similar, fast evolving and bright secondary maxima (SdM) appeared in succession. Only very few other novae have presented so clear secondary maxima. Both the primary maximum and all SdM occurred at later times with increasing wavelengths, by amounts in agreement with expectations from fireball expansions. The radiative energy released during SdM declined following an exponential pattern, while the breadth of individual SdM and the time interval between them widened. Emission lines remained sharp (FWHM~1000 km/s) throughout the whole nova evolution, with the exception of a broad pedestal with a trapezoidal shape (extending for 3600 km/sec at the top and 4500 km/sec at the bottom) which was only seen during the advanced decline from SdM maxima and was absent in between SdM. V5588 Sgr at maximum light displayed a typical FeII-class spectrum which did not evolve into a nebular stage. About 10 days into the decline from primary maximum, a typical high-ionization He/N-class spectrum appeared and remained visible simultaneously with the FeII-class spectrum, qualifying V5588 Sgr as a rare "hybrid" nova. While the FeII-class spectrum faded into oblivion, the He/N-class spectrum developed strong [FeX] coronal lines.

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arXiv2014

Optical/IR studies of Be stars in NGC 6834 with emphasis on two specific stars

We present optical and infrared photometric and spectroscopic studies of two Be stars in the 70--80-Myr-old open cluster NGC 6834. NGC 6834(1) has been reported as a binary from speckle interferometric studies whereas NGC 6834(2) may possibly be a gamma Cas-like variable. Infrared photometry and spectroscopy from the United Kingdom Infrared Telescope (UKIRT), and optical data from various facilities are combined with archival data to understand the nature of these candidates. High signal-to-noise near-IR spectra obtained from UKIRT have enabled us to study the optical depth effects in the hydrogen emission lines of these stars. We have explored the spectral classification scheme based on the intensity of emission lines in the $H$ and $K$ bands and contrasted it with the conventional classification based on the intensity of hydrogen and helium absorption lines. This work also presents hitherto unavailable UBV CCD photometry of NGC 6834, from which the evolutionary state of the Be stars is identified.

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arXiv2014

V838 Monocerotis: the central star and its environment a decade after outburst

Aims. V838 Monocerotis erupted in 2002, brightened in a series of outbursts, and eventually developed a spectacular light echo. A very red star emerged a few months after the outburst. The whole event has been interpreted as the result of a merger. Methods. We obtained near-IR and mid-IR interferometric observations of V838 Mon with the AMBER and MIDI recombiners located at the Very Large Telescope Interferometer (VLTI) array. The MIDI two-beam observations were obtained with the 8m Unit Telescopes between October 2011 and February 2012. The AMBER three-beam observations were obtained with the compact array (B$\leq$m) in April 2013 and the long array (B$\leq$140m) in May 2014, using the 1.8m Auxiliary Telescopes. Results. A significant new result is the detection of a compact structure around V838 Mon, as seen from MIDI data. The extension of the structure increases from a FWHM of 25 mas at 8 μm to 70 mas at 13 μm. At the adopted distance of D = 6.1 $\pm$ 0.6 kpc, the dust is distributed from about 150 to 400 AU around V838 Mon. The MIDI visibilities reveal a flattened structure whose aspect ratio increases with wavelength. The major axis is roughly oriented around a position angle of -10 degrees, which aligns with previous polarimetric studies reported in the literature. This flattening can be interpreted as a relic of the 2002 eruption or by the influence of the currently embedded B3V companion. The AMBER data provide a new diameter for the pseudo-photosphere, which shows that its diameter has decreased by about 40% in 10yrs, reaching a radius R$_*$ = 750 $\pm$ 200 R$_{\odot}$ (3.5 $\pm$ 1.0 AU). Conclusions. After the 2002 eruption, interpreted as the merging of two stars, it seems that the resulting source is relaxing to a normal state. The nearby environment exhibits an equatorial over-density of dust up to several hundreds of AU.

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arXiv2014

The peculiar extinction law of SN2014J measured with The Hubble Space Telescope

The wavelength-dependence of the extinction of Type Ia SN2014J in the nearby galaxy M82 has been measured using UV to near-IR photometry obtained with the Hubble Space Telescope, the Nordic Optical Telescope, and the Mount Abu Infrared Telescope. This is the first time that the reddening of a SN Ia is characterized over the full wavelength range of $0.2$-$2$ microns. A total-to-selective extinction, $R_V\geq3.1$, is ruled out with high significance. The best fit at maximum using a Galactic type extinction law yields $R_V = 1.4\pm0.1$. The observed reddening of SN2014J is also compatible with a power-law extinction, $A_λ/A_V = \left( λ/ {λ_V} \right)^{p}$ as expected from multiple scattering of light, with $p=-2.1\pm0.1$. After correction for differences in reddening, SN2014J appears to be very similar to SN2011fe over the 14 broad-band filter light curves used in our study.

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arXiv2014

Early Observations and Analysis of the Type Ia SN 2014J in M82

We present optical and near infrared (NIR) observations of the nearby Type Ia SN 2014J. Seventeen optical and twenty-three NIR spectra were obtained from 10 days before ($-$10d) to 10 days after (+10d) the time of maximum $B$-band brightness. The relative strengths of absorption features and their patterns of development can be compared at one day intervals throughout most of this period. Carbon is not detected in the optical spectra, but we identify CI $λ$ 1.0693 in the NIR spectra. We find that MgII lines with high oscillator strengths have higher initial velocities than other MgII lines. We show that the velocity differences can be explained by differences in optical depths due to oscillator strengths. The spectra of SN 2014J show it is a normal SN Ia, but many parameters are near the boundaries between normal and high-velocity subclasses. The velocities for OI, MgII, SiII, SII, CaII and FeII suggest that SN 2014J has a layered structure with little or no mixing. That result is consistent with the delayed detonation explosion models. We also report photometric observations, obtained from $-$10d to +29d, in the $UBVRIJH$ and $K_s$ bands. SN 2014J is about 3 magnitudes fainter than a normal SN Ia at the distance of M82, which we attribute to extinction in the host. The template fitting package SNooPy is used to interpret the light curves and to derive photometric parameters. Using $R_V$ = 1.46, which is consistent with previous studies, SNooPy finds that $A_V = 1.80$ for $E(B-V)_{host}=1.23 \pm 0.01$ mag. The maximum $B$-band brightness of $-19.19 \pm 0.10$ mag was reached on February 1.74 UT $ \pm 0.13$ days and the supernova had a decline parameter of $Δm_{15}=1.11 \pm 0.02$ mag.

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arXiv2014

The rise of SN2014J in the nearby galaxy M82

We report on the discovery of SN2014J in the nearby galaxy M82. Given its proximity, it offers the best opportunity to date to study a thermonuclear supernova over a wide range of the electromagnetic spectrum. The first set of optical, near-IR and mid-IR observations of SN2014J, orchestrated by the intermediate Palomar Transient Factory (iPTF), show that SN2014J is a spectroscopically normal Type Ia supernova, albeit exhibiting high-velocity features in its spectrum and heavily reddened by dust in the host galaxy. Our earliest detections start just hours after the fitted time of explosion. We use high-resolution optical spectroscopy to analyze the dense intervening material and do not detect any evolution in the resolved absorption features during the lightcurve rise. Similarly to other highly reddened Type Ia supernovae, a low value of total-to-selective extinction, Rv < 2, provides the best match to our observations. We also study pre-explosion optical and near-IR images from HST with special emphasis on the sources nearest to the SN location.

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arXiv2014

Near-IR studies of recurrent nova V745 Scorpii during its 2014 outburst

The recurrent nova (RN) V745 Scorpii underwent its third known outburst on 2014 February 6. Infrared monitoring of the eruption on an almost daily basis, starting from 1.3d after discovery, shows the emergence of a powerful blast wave generated by the high velocity nova ejecta exceeding 4000 kms$^{-1}$ plowing into its surrounding environment. The temperature of the shocked gas is raised to a high value exceeding 10$^{8}$K immediately after outburst commencement. The energetics of the outburst clearly surpass those of similar symbiotic systems like RS Oph and V407 Cyg which have giant secondaries. The shock does not show a free-expansion stage but rather shows a decelerative Sedov-Taylor phase from the beginning. Such strong shockfronts are known to be sites for $γ$ ray generation. V745 Sco is the latest nova, apart from five other known novae, to show $γ$ ray emission. It may be an important testbed to resolve the crucial question whether all novae are generically $γ$ ray emitters by virtue of having a circumbinary reservoir of material that is shocked by the ejecta rather than $γ$ ray generation being restricted to only symbiotic systems with a shocked red giant (RG) wind. The lack of a free-expansion stage favors V745 Sco to have a density enhancement around the white dwarf (WD), above that contributed by a RG wind. Our analysis also suggests that the WD in V745 Sco is very massive and a potential progenitor for a future SN Ia explosion.

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arXiv2013

Near-infrared spectroscopic and photometric evolution of Nova V476 Scuti -- a nova that formed optically thin dust

We present results of near-infrared (near-IR) $JHK$ (1.07 - 2.5 $μ$m) spectroscopic and photometric observations of Nova V476 Scuti (V476 Sct) which was discovered in outburst in 2005 September. The near-IR observations of the nova presents the evolution of the post-maxima spectra and near-IR light curve. The spectra of V476 Sct, observed on 9 different epochs, show prominent lines due to HI, OI, CI and NI. Based on the IR spectral signatures we independently identify it as a Fe II type of nova, consistent with the same classification obtained from optical spectra. A detailed identification of the observed spectral lines is presented. The near-IR $JHK$ light curve extending for a period of about 59 days after outburst clearly shows the formation of a optically thin dust shell, a phenomenon which is not commonly observed in novae. By fitting black body curves to the spectral energy distributions (SEDs) the temperatures of the dust shell on different epochs have been estimated. Dust formation in V476 Sct is consistent with the presence of lines of elements with low-ionization potential like Na and Mg in the early spectra which had earlier been suggested by us to be potential indicators of dust formation at a later phase in a nova's development.

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arXiv2013

Near-infrared properties of classical novae: a perspective gained from Mount Abu Infrared Observatory

We review the near-infrared properties of classical novae in the J, H and K bands at wavelengths between 1.08 to 2.4 micron. A classification system exists for the early post-outburst optical spectra of novae on the basis of the strength of group of non-hydrogen emission lines. A similar scheme for the near-infrared regime, which is not available at present, is presented here. In the optical system there are two principal classes, namely, "Fe II" and "He/N" for novae with either prominent Fe II lines or prominent "He/N" lines. There is also a small subset of the hybrid Fe IIb type. From spectroscopic observations we show the differences and similarities between these classes of novae in the near-infrared. The spectral lines common to the two principal classes arise from H, He, N and O. However, the near-IR features that separate these two classes are the numerous, and often strong, Carbon lines which are seen only in the spectra of the Fe II class of novae. The dust formation process in novae is discussed based on broad-band observations. The first-overtone carbon monoxide (CO) detections in novae are analyzed to understand the formation and evolution of this molecule in the nova ejecta and to discuss the observed 12C/13C ratio.

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arXiv2013

Nova KT Eri 2009: Infrared studies of a very fast and small amplitude He/N nova

We present near-infrared spectroscopic and photometric observations of the nova KT Eridani taken during the first 100 days following its discovery in 2009 November. The JHK spectra of the object have been taken from the Mount Abu Infrared Observatory using the Near-Infrared Imager/Spectrometer. The spectra, typical of the He/N class novae, show strong He I emission lines together with H I and O I emission features. The H I, Pa-beta and Br-gamma spectral lines and the He I line at 2.0581 micron show broad wings with a relatively narrow central component. The broad wings extend to 1900 km/s while the central component has FWHM of 2100 km/s. The V and near-infrared JHK light curves show an additional small amplitude outburst near 40 days after optical maximum. The distance to the nova d = 6.3 +/- 0.1 kpc is derived using the MMRD relation and the estimated value of t2 = 5.7 +/- 0.3 days. The small value of t2 places KT Eri in the class of very fast novae. Using the value of the distance to the nova d, we estimate the height of the nova to be z = 3.3 +/- 0.1 kpc below the galactic plane. We have also calculated the upper limit for the ejecta mass for KT Eri to be in the range 2.4-7.4 x 10^(-5) Msun. Kinematic evidence is presented from the shape of the line profiles for a possible bipolar flow. We analyze the temporal evolution of the continuum and also discuss the possibility of KT Eri being a recurrent nova.

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arXiv2013

Studies of the Be star X Persei during a bright infrared phase

We present multi-epoch photometric and spectroscopic near-infrared observations of the Be star X Persei in the JHK bands covering the wavelength range 1.08 to 2.35 micron. Combining results from our earlier studies with the present observations, it is shown that the equivalent widths and line fluxes of the prominent HI and HeI lines anti-correlate with the strength of the adjacent continuum. That is, during the span of the observations, the JHK broad band fluxes increase while the emission equivalent width values of the HI and HeI lines decrease (the lines under consideration being the Paschen and Brackett lines of hydrogen and the 1.7002 micron and 2.058 micron lines of helium). Such an anti-correlation effect is not commonly observed in Be stars in the optical; in the infrared this may possibly be the first reported case of such behavior. We examine different mechanisms that could possibly cause it and suggest that it could originate due to a radiatively warped, precessing circumstellar disk. It is also shown that during the course of our studies X Per evolved to an unprecedented state of high near-IR brightness with J, H, K magnitudes of 5.20, 5.05 and 4.84 respectively.

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arXiv2013

V496 Scuti: Detection of CO emission and dust shell in a moderately fast Fe II nova

We present near-infrared and optical observations of moderately fast FeII-class Nova Scuti 2009 (V496 Sct) covering various phases; pre-maximum, early decline and nebular, during the first 10 months after its discovery followed by limited observations up to 2011 April. In the initial phase the nova spectra show prominent P Cygni profiles and later all the lines are seen in emission. The notable feature of the near-IR spec- tra in the early decline phase is the rare presence of the first overtone bands of carbon monoxide (CO) in emission. The IR spectra show clear dust formation in the expand- ing ejecta at later phase about 150 days after the peak brightness. The presence of lines of elements with low ionization potentials like Na and Mg in the early IR spectra and the detection of CO bands in emission and the dust formation in V496 Sct represents a complete expected sequence in the dust formation in nova ejecta. The light curve shows a slow rise to the maximum and a slow decline indicating a prolonged mass loss. This is corroborated by the strengthening of P Cygni profiles during the first 30 days. The broad and single absorption components seen in many lines in the optical spectra at the time of discovery are replaced by two sharper components in the spectra taken close to the optical maximum brightness. These sharp dips seen in the P Cygni absorption components of Fe II and H I lines during the early decline phase show increasing outflow velocities. The onset of the nebular phase is evident from the optical spectra in 2010 March. During the nebular phase, several emission lines display saddle-like profiles. In the nebular stage, the observed fluxes of [O III] and H lines are used to estimate the electron number densities and the mass of the ejecta. The optical spectra show that the nova is evolved in the P_fe A_o spectral sequence.

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arXiv2012

The expanding dusty bipolar nebula around the nova V1280 Sco

V1280 Sco is one of the slowest dust-forming nova ever historically observed. We performed multi-epoch high-spatial resolution observations of the circumstellar dusty environment of V1280 Sco to investigate the level of asymmetry of the ejecta We observed V1280 Sco in 2009, 2010 and 2011 using unprecedented high angular resolution techniques. We used the NACO/VLT adaptive optics system in the J, H and K bands, together with contemporaneous VISIR/VLT mid-IR imaging that resolved the dust envelope of V1280 Sco, and SINFONI/VLT observations secured in 2011. We report the discovery of a dusty hourglass-shaped bipolar nebula. The apparent size of the nebula increased from 0.30" x 0.17" in July 2009 to 0.64" x 0.42" in July 2011. The aspect ratio suggests that the source is seen at high inclination. The central source shines efficiently in the K band and represents more than 56+/-5% of the total flux in 2009, and 87+/-6% in 2011. A mean expansion rate of 0.39+/-0.03 mas per day is inferred from the VISIR observations in the direction of the major axis, which represents a projected upper limit. Assuming that the dust shell expands in that direction as fast as the low-excitation slow ejecta detected in spectroscopy, this yields a lower limit distance to V1280 Sco of 1kpc; however, the systematic errors remain large due to the complex shape and velocity field of the dusty ejecta. The dust seems to reside essentially in the polar caps and no infrared flux is detected in the equatorial regions in the latest dataset. This may imply that the mass-loss was dominantly polar.

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arXiv2012

V496 Scuti: An Fe II nova with dust shell accompanied by CO emission

We present near-infrared and optical observations of the nova V496 Scuti 2009 covering various phases - pre-maximum, early decline and nebular - during the first 10 months of its discovery followed by limited observations in early part of 2011 April. The spectra follow the evolution of the nova when the lines had strong P Cygni profiles to a phase dominated by prominent emission lines. The notable feature of the near-IR spectra in the early decline phase is the rare presence of first overtone bands of carbon monoxide in emission. Later about 150 days after the peak brightness the IR spectra show clear dust formation in the expanding ejecta. Dust formation in V496 Sct is consistent with the presence of lines of elements with low ionization potentials like Na and Mg in the early spectra and the detection of CO bands in emission. The light curve shows a slow rise to the maximum and a slow decline indicating a prolonged mass loss. This is corroborated by the strengthening of P Cygni profiles during the first 30 days. In the spectra taken close to the optical maximum brightness, the broad and single absorption component seen at the time of discovery is replaced by two sharper components. During the early decline phase two sharp dips that show increasing outflow velocities are seen in the P Cygni absorption components of Fe II and H I lines. The spectra in 2010 March showed the onset of the nebular phase. Several emission lines display saddle-like profiles during the nebular phase. In the nebular stage the observed fluxes of [O III] and H-beta lines are used to estimate the electron number densities and the mass of the ejecta. The optical spectra show that the nova evolved in the P_fe A_o spectral sequence. The physical conditions in the ejecta are estimated. The absolute magnitude and the distance to the nova are estimated to be M_V = -7.0 +/- 0.2 and d = 2.9 +/- 0.3 kpc respectively.

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arXiv2012

The A-X infrared bands of Aluminum Oxide in stars: search and new detections

We describe a search for the A-X infrared bands of AlO with a view to better understand the characteristics of this radical. These bands are infrequently encountered in astronomical sources but surprisingly were very prominent in the spectra of two well-known, nova-like variables (V838 Mon and V4332 Sgr) thereby motivating us to explore the physical conditions necessary for their excitation. In this study, we present the detection of A-X bands in the spectra of 13 out of 17 stars, selected on the basis of their J-K colors as potential candidates for detection of these bands. The majority of the AlO detections are in AGB stars viz. 9 OH/IR stars, 2 Mira variables and 2 bright infrared sources. Our study shows that the A-X bands are fairly prevalent in sources with low temperature and O-rich environments. Interesting variation in strength of the AlO bands in one of the sources (IRAS 18530+0817) is reported and the cause for this is examined. Possible applications of the present study are discussed in terms of the role of AlO in alumina dust formation, the scope for estimating the radioactive $^{26}$Al content in AGB stars from the A-X bands, and providing possible targets for further mm/radio studies of AlO which has recently been discovered at millimeter wavelengths.

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arXiv2012

Infrared observations of the recurrent nova T Pyxidis: ancient dust basks in the warm glow of the 2011 outburst

We present Spitzer Space Telescope and Herschel Space Observatory infrared observations of the recurrent nova T Pyx during its 2011 eruption, complemented by ground-base optical-infrared photometry. We find that the eruption has heated dust in the pre-existing nebulosity associated with T Pyx. This is most likely interstellar dust swept up by T Pyx - either during previous eruptions or by a wind - rather than the accumulation of dust produced during eruptions.

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