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Register a new userOptical/IR studies of Be stars in NGC 6834 with emphasis on two specific stars
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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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In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs.
We present a spectroscopic study of 150 Classical Be stars in 39 open clusters using medium resolution spectra in the wavelength range 3800 - 9000 AA. 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 AA. 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 AA. We have identified short and long-term line profile variability in some candidate stars through repeated observations.
In this study, we analyze the emission lines of different species present in 118 Galactic field classical Be stars in the wavelength range of 3800 - 9000 AA. We re-estimated the extinction parameter (A$_V$) for our sample stars using the newly available data from Gaia DR2 and suggest that it is important to consider A$_V$ while measuring the Balmer decrement (i.e. $D_{34}$ and $D_{54}$) values in classical Be stars. Subsequently, we estimated the Balmer decrement values for 105 program stars and found that $approx$ 20% of them show $D_{34}$ $geq$ 2.7, implying that their circumstellar disc are generally optically thick in nature. One program star, HD 60855 shows H$alpha$ in absorption -- indicative of discless phase. From our analysis, we found that in classical Be stars, H$alpha$ emission equivalent width values are mostly lower than 40 AA, which agrees with that present in literature. Moreover, we noticed that a threshold value of $sim$ 10 AA~of H$alpha$ emission equivalent width is necessary for Fe{sc ii} emission to become visible. We also observed that emission line equivalent widths of H$alpha$, P14, Fe{sc ii} 5169 and O{sc i} 8446 AA~for our program stars tend to be more intense in earlier spectral types, peaking mostly near B1-B2. Furthermore, we explored various formation regions of Ca{sc ii} emission lines around the circumstellar disc of classical Be stars. We suggest the possibility that Ca{sc ii} triplet emission can originate either in the circumbinary disc or from the cooler outer regions of the disc, which might not be isothermal in nature.
We investigate the short-term optical variability of two gamma Cas analogs, pi Aqr and BZ Cru, thanks to intensive ground-based spectroscopic and space-borne photometric monitorings. For both stars, low-amplitude (mmag) coherent photometric variability is detected. The associated signals display long-term amplitude variations, as in other Be stars. However, these signals appear at high frequencies, especially in pi Aqr, indicating p-modes with a high degree l, a quite unusual feature amongst Be stars. While BZ Cru presents only low-level spectral variability, without clear periodicity, this is not the case of pi Aqr. In this star, the dominant photometric frequencies, near ~12/d, are confirmed spectroscopically in separate monitorings taken during very different disk activity levels ; the spectroscopic analysis suggests a probable tesseral nature for the mode.
We present new UVES spectra of a sample of 14 mostly cool unevolved stars with planetary companions with the aim of studying possible differences in Be abundance with respect to stars without detected planets. We determine Be abundances for these stars that show an increase in Be depletion as we move to lower temperatures. We carry out a differential analysis of spectra of analog stars with and without planets to establish a possible difference in Be content. While for hot stars no measurable difference is found in Be, for the only cool (Teff ~ 5000 K) planet host star with several analogs in the sample we find enhanced Be depletion by 0.25 dex. This is a first indication that the extra-depletion of Li in solar-type stars with planets may also happen for Be, but shifted towards lower temperatures (Teff < 5500 K) due to the depth of the convective envelopes. The processes that take place in the formation of planetary systems may affect the mixing of material inside their host stars and hence the abundances of light elements.
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