Abstract Details
| Name: V.Venkataraman Affiliation: Physical Research Laboratory Conference ID: ASI2016_681 Title : Infrared investigations of circumstellar matter Authors and Co-Authors : V.Venkataraman Physical Research Laboratory Abstract Type : Oral Abstract Category : Thesis Abstract : Circumstellar matter enshrouds both the protostars which accrete from their surroundings as well as evolved stars which undergo significant mass loss, leading in both cases to an excess emission in the infrared (IR). The thesis presents infrared investigations of circumstellar matter around eruptive young pre-main sequence (PMS) stars and evolved low and intermediate mass stars. The eruptive events in PMS stars are believed to be caused by enhanced disk mass accretion. We report an unprecedented high cadence IR monitoring of V1647 Ori, an enigmatic eruptive variable star that attracted significant attention over the last decade. It experienced two episodes of outbursts during 2004 and 2008. The nature of the outburst and its similarity to the PMS eruptive events that are classified as FUors and EXors depending on the duration of the outburst makes it distinctive. The physical and circumstellar parameters obtained by modeling the spectral energy distribution (SED) during the outburst and quiescent phases showed marked differences. The disk mass and disk accretion rate during the outburst phase are significantly greater than those in the quiescent phase. The envelope mass and accretion rate are higher during quiescent phase. The second part of the thesis deals with the photometric and spectroscopic study of evolved stages of stars namely Asymptotic Giant Branch (AGB) and post-AGB (PAGB)/ Proto-planetary nebulae (PPNe) stages. The near-infrared (NIR) spectra of AGB stars showed CO vibration-rotation lines of first and second overtone bands. The equivalent width (EW) of the second overtone band of CO correlated with IR colors, suggesting a possible relationship with mass-loss processes. The NIR spectra of a sample of PAGB stars showed short-term spectral variability that is indicative of ongoing episodic mass loss. Cooler PAGB stars showed variation in CO first overtone lines, while hotter stars showed variability in HI lines. The mid-IR spectra of AGB stars obtained from the infrared space observatory (ISO) archives showed the presence of silicates either in absorption or emission. Also, the spectra showed the absence of polycyclic aromatic hydrocarbons (PAHs) in all our sample stars. The mid-IR spectra of PAGB/PPNe candidates obtained from the Spitzer data archive showed different spectral features that include emissions from PAHs. A trend of dependency was noticed between the EWs of various vibrational modes of PAHs and their photospheric temperature obtained by modeling the SED. The PAGB/PPNe mass loss rates obtained from the SED models are higher than the AGB mass loss rates. The significant difference between the AGB and PAGB/PPNe candidates lies in the SED of these sources where PAGB/PPNe sources have detached shells leading to a double humped SED while AGB candidates show a smooth SED due to the on-going mass loss. A few transition objects with low-excitation line emission of [NeII] were identified among PAGB/PPNe sample, which indicates their advanced stage of evolution. IR vibration modes of fullerenes (C60) were detected for the first time in the PAGB star IRAS 21546+4721 making it one of the very few evolved stars that showed the presence of fullerenes. Thesis supervisor: Prof. B.G.Anandarao |