arXiv subjects

Blesson Mathew

Publications and source records attributed to Blesson Mathew.

82 records · Page 5Linked to original sources

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

A study of the role of Lyman β fluorescence on OI line strengths in Be stars

The possibility of the Lyβ fluorescence mechanism being operational in classical Be stars and thereby contributing to the strength of the OI 8446 Å line has been recognized for long. However this supposition needs to be quantified by comparing observed and predicted OI line ratios. In the present work, optical and near-infrared spectra of classical Be stars are presented. We analyse the observed strengths of the OI 7774, 8446, 11287 and 13165 Å lines which have been theoretically proposed as diagnostics for identifying the excitation mechanism. We have considered and examined the effects of Lyβ fluorescence, collisional excitation, recombination and continuum fluorescence on these OI line strengths. From our analysis it appears that the Lyβ fluorescence process is indeed operative in Be stars.

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arXiv2012

Infrared studies of the Be star X Per

Photometric and spectroscopic results are presented for the Be star X Per/HD 24534 from near-infrared monitoring in 2010-2011. The star is one of a sample of selected Be/X-ray binaries being monitored by us in the near-IR to study correlations between their X ray and near-IR behaviour. Comparison of the star's present near-IR magnitudes with earlier records shows the star to be currently in a prominently bright state with mean J, H, K magnitudes of 5.49, 5.33 and 5.06 respectively. The JHK spectra are dominated by emission lines of HeI and Paschen and Brackett lines of HI. Lines of OI 1.1287 and 1.3165 micron are also present and their relative strength indicates, since OI 1.1287 is stronger among the two lines, that Lyman beta fluorescence plays an important role in their excitation. Recombination analysis of the HI lines is done which shows that the Paschen and Brackett line strengths deviate considerably from case B predictions. These deviations are attributed to the lines being optically thick and this supposition is verified by calculating the line center optical depths predicted by recombination theory. Similar calculations indicate that the Pfund and Humphrey series lines should also be expected to be optically thick which is found to be consistent with observations reported in other studies. The spectral energy distribution of the star is constructed and shown to have an infrared excess. Based on the magnitude of the IR excess, which is modeled using a free-free contribution from the disc, the electron density in the disc is estimated and shown to be within the range of values expected in Be star discs.

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arXiv2012

Studies of a possible new Herbig Ae/Be star in the open cluster NGC 7380

We present a study of the star 2MASS J22472238+5801214 with the aim of identifying its true nature which has hitherto been uncertain. This object, which is a member of the young cluster NGC 7380, has been variously proposed to be a Be star, a D-type symbiotic and a Herbig Ae/Be star in separate studies. Here we present optical spectroscopy, near-IR photometry and narrow band H-alpha imaging of the nebulosity in its environment. Analysis of all these results, including the spectral energy distribution constructed from available data, strongly indicate the source to be a Herbig Ae/Be star. The star is found to be accompanied by a nebulosity with an interesting structure. A bow shock shaped structure, similar to a cometary nebula, is seen very close to the star with its apex oriented towards the photoionizing source of this region (i.e. the star DH Cep). An interesting spectroscopic finding, from the forbidden [SII] 6716, 6731 Åand [OI] 6300 Ålines, is the detection of a blue-shifted high velocity outflow (200 +/- 50 km/s) from the star.

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arXiv2011

Near-infrared Observations of Be/X-ray Binary Pulsar A0535+262

We present results obtained from an extensive near-infrared spectroscopic and photometric observations of the Be/X-ray binary A0535+262/HDE 245770 at different phases of its ~111 day orbital period. This observation campaign is a part of the monitoring programme of selective Be/X-ray binary systems aimed at understanding the X-ray and near-IR properties at different orbital phases, especially during the periastron passage of the neutron star. The near-IR observations were carried out using the 1.2 m telescope at Mt. Abu IR observatory. Though the source was relatively faint for spectroscopic observations with 1.2 m telescope, we monitored the source during the 2011 February--March giant outburst to primarily investigate whether any drastic changes in the near-IR JHK spectra take place at the periastron passage. Changes of such a striking nature were expected to be detectable in our spectra. Photometric observations of the Be star show a gradual and systematic fading in the JHK light curves since the onset of the X-ray outburst that could suggest a mild evacuation/truncation of the circumstellar disc of the Be companion. Near-IR spectroscopy of the object shows that the JHK spectra are dominated by the emission lines of hydrogen Brackett and Paschen series and HeI lines at 1.0830, 1.7002 and 2.0585 micron. The presence of all hydrogen emission lines in the JHK spectra, along with the absence of any significant change in the continuum of the Be companion during X-ray quiescent and X-ray outburst phases suggest that the near-IR line emitting regions of the disc are not significantly affected during the X-ray outburst.

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arXiv2011

Optical spectroscopy of Classical Be stars in open clusters

We present a spectroscopic study of 150 Classical Be stars in 39 open clusters using medium resolution spectra in the wavelength range 3800 - 9000 Å. One-third of the sample (48 stars in 18 clusters) has been studied for the first time. All these candidates were identified from an extensive survey of emission stars in young open clusters using slitless spectroscopy (Mathew et al. 2008). This large data set covers CBe stars of various spectral types and ages found in different cluster environments in largely northern open clusters, and is used to study the spectral characteristics of CBe stars in cluster environments. About 80% of CBe stars in our sample have H-alpha equivalent width in the range -1 to -40 Å. About 86% of the surveyed CBe stars show Fe II lines. The prominent Fe II lines in our surveyed stars are 4584, 5018, 5169, 5316, 6318, 6384, 7513 and 7712 Å. We have identified short and long-term line profile variability in some candidate stars through repeated observations.

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arXiv2008

Optical and near-IR photometric study of the open cluster NGC 637 and NGC 957

We present UBVRI CCD photometry in the region of the open clusters NGC 637 and NGC 957. The radii are found to be 4.2 arcmin and 4.3 arcmin. Their reddenings E(B-V) are 0.64+/-0.05 mag and 0.71+/-0.05 mag and their distances, from main sequence fitting are 2.5+/-0.2 kpc and 2.2+/-0.2 kpc. Comparison with Z=0.02 isochrones leads to an age of 10+/-5 Myr for both clusters. Combining our photometry with 2MASS JHK shows the reddening law in these directions to be normal. Mass function slopes of x=1.65+/-0.20 and 1.31+/-0.50 are derived for the clusters, both of which are found to be dynamically relaxed. Spectral and photometric characteristics of three Be stars, two in NGC 957 and one (newly discovered) in NGC 637 indicate them to be of Classical Be type.

astro-ph↗

arXiv2008

Pre-main sequence stars, emission stars and recent star formation in the Cygnus Region

The recent star formation history in the Cygnus region is studied using 5 clusters (IC 4996, NGC 6910, Berkeley 87, Biurakan 2 and Berkeley 86). The optical data from the literature are combined with the 2MASS data to identify the pre-main sequence (pre-MS) stars as stars with near IR excess. We identified 93 pre-MS stars and 9 stars with H$_α$ emission spectra. The identified pre-MS stars are used to estimate the turn-on age of the clusters. The duration of star formation was estimated as the difference between the turn-on and the turn-off age. We find that, NGC 6910 and IC 4996 have been forming stars continuously for the last 6 -- 7 Myr, Berkeley 86 and Biurakan 2 for 5 Myr and Berkeley 87 for the last 2 Myr. This indicates that the Cygnus region has been actively forming stars for the last 7 Myr, depending on the location. 9 emission line stars were identified in 4 clusters, using slit-less spectra (Be 87 - 4 stars; Be 86 - 2 stars, NGC 6910 - 2 stars and IC 4996 - 1 star). The individual spectra were obtained and analysed to estimate stellar as well as disk properties. All the emission stars are in the MS, well below the turn-off, in the core hydrogen burning phase. These stars are likely to be Classical Be (CBe) stars. Thus CBe phenomenon can be found in very young MS stars which are just a few (2 -- 7) Myrs old. This is an indication that CBe phenomenon need not be an evolutionary effect.

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arXiv2008

Be phenomenon in open clusters: Results from a survey of emission-line stars in young open clusters

Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and their evolutionary state. 207 open star clusters were observed using slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20% clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0--10 and 20--30 Myr age bin ($\sim$ 40% each) and in the other age bins, this fraction ranges between 10% -- 25%, upto 80 Myr. We have used optical colour magnitude diagram (CMD) along with Near-IR Colour-Colour diagram (NIR CCDm) to classify the emission stars into Classical Be (CBe) stars and Herbig Be (HBe) stars. Most of the emission stars in our survey belong to CBe class ($\sim$ 92%) while a few are HBe stars ($\sim$ 6%) and HAe stars ($\sim$1%). The CBe stars are located all along the MS in the optical CMDs of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. Most of the clusters which contain emission stars are found in Cygnus, Perseus & Monoceros region of the Galaxy, which are locations of active star formation. The distribution of CBe stars as a function of spectral type shows peaks at B1-B2 and B6-B7. Our results indicate there could be two mechanisms responsible for the CBe phenomenon. Some are born CBe stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to CBe stars, as indicates by the enhancement in the fraction of clusters with CBe stars in the 20-30 Myr age bin.

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arXiv2006

NGC 7419: A young open cluster with a number of very young intermediate mass pre-MS stars

We present a photometric and spectroscopic study of the young open cluster NGC 7419, which is know to host a large number of classical Be stars for reasons not well understood. Based on CCD photometric observations of 327 stars in UBV passbands, we estimated the cluster parameters as, reddening E(B-V) = 1.65 +/- 0.15 mag and distance = 2900 +/- 400 pc. The turn off age of the cluster was estimated as 25 +/- 5 Myr using isochrone fits. UBV data of the stars were combined with JHK data from 2MASS and were used to create the near infrared (NIR) (J-H) vs (H-K) colour-colour diagram. A large fraction of stars (42%) was found to have NIR excess and their location in the diagram was used to identify them as intermediate mass pre-MS stars. The isochrone fits to pre-MS stars in the optical colour-magnitude diagram showed that the turn-on age of the cluster is 0.3 - 3 Myr. This indicates that there has been a recent episode of star formation in the vicinity of the cluster. Slit-less spectra were used to identify 27 stars which showed H-alpha in emission in the field of the cluster, of which 6 are new identifications. All these stars were found to show NIR excess and located closer to the region populated by Herbig Ae/Be stars in the (J-H) vs (H-K) diagram. Slit spectra of 25 stars were obtained in the region 3700A - 9000A. The spectral features were found to be very similar to those of Herbig Be stars. Those stars were found to be more reddened than the main sequence stars by 0.4 mag on an average. Thus the emission line stars found in this cluster are more similar to the Herbig Be type stars where the circumstellar material is the remnant of the accretion disk.

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