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Register a new userOptical spectroscopy of Be/gamma-ray binaries
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Added by
Radoslav K. Zamanov
Publication date
2016
fields
Physics
and research's language is
English
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We report optical spectroscopic observations of the Be/gamma-ray binaries LSI+61303, MWC 148 and MWC 656. The peak separation and equivalent widths of prominent emission lines (H-alpha, H-beta, H-gamma, HeI, and FeII) are measured. We estimated the circumstellar disc size, compared it with separation between the components, and discussed the disc truncation. We find that in LSI+61303 the compact object comes into contact with the outer parts of the circumstellar disc at periastron, in MWC 148 the compact object goes deeply into the disc during the periastron passage, and in MWC 656 the black hole is accreting from the outer parts of the circumstellar disc along the entire orbit. The interstellar extinction was estimated using interstellar lines. The rotation of the mass donors appears to be similar to the rotation of the mass donors in Be/X-ray binaries. We suggest that X-ray/optical periodicity of about 1 day deserves to be searched for.
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Optical and near-infrared observations are compiled for the three gamma-ray binaries hosting Be stars: PSR B1259-63, LSI+61 303, and HESS J0632+057. The emissions from the Be disk are considered to vary according to the changes in its structure, some of which are caused by interactions with the compact object (e.g., tidal forces). Due to the high eccentricity and large orbit of these systems, the interactions -- and, hence the resultant observables -- depend on the orbital phase. To explore such variations, multi-band photometry and linear polarization were monitored for the three considered systems, using two 1.5 m-class telescopes: IRSF at the South African Astronomical Observatory and Kanata at the Higashi-Hiroshima Observatory.
Using TESS photometry and Rozhen spectra of the Be/gamma-ray binaries MWC 148 and MWC 656, we estimate the projected rotational velocity ($ {v} sin i$), the rotational period (P$_{rm rot}$), radius (R$_{rm 1}$), and inclination ($i$) of the mass donor. For MWC 148 we derive P$_{rm rot} = 1.10 pm 0.03$~d, R$_{rm 1}= 9.2 pm 0.5$~R$_odot$, $i = 40^circ pm 2^circ$, and $ {v} sin i =272 pm 5$~km~s$^{-1}$. For MWC 656 we obtain P$_{rm rot} = 1.12 pm 0.03$~d, R$_{rm 1}= 8.8 pm 0.5$~R$_odot$, $i = 52^circ pm 3^circ$, and $ {v} sin i =313 pm 3$~km~s$^{-1}$. For MWC 656 we also find that the rotation of the mass donor is coplanar with the orbital plane.
In this study, we analyze the emission lines of different species present in 118 Galactic field classical Be stars in the wavelength range of 3800 - 9000 AA. We re-estimated the extinction parameter (A$_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.
Context: The identification of long-gamma-ray-bursts (LGRBs) is still uncertain, although the collapsar engine of fast-rotating massive stars is gaining a strong consensus. Aims: We propose that low-metallicity Be and Oe stars, which are massive fast rotators, as potential LGRBs progenitors. Methods: We checked this hypothesis by 1) testing the global specific angular momentum of Oe/Be stars in the ZAMS with the SMC metallicity, 2) comparing the ZAMS ($Omega/Omega_{rm c},M/M_{odot}$) parameters of these stars with the area predicted theoretically for progenitors with metallicity $Z=0.002$, and 3) calculating the expected rate of LGRBs/year/galaxy and comparing them with the observed ones. To this end, we determined the ZAMS linear and angular rotational velocities for SMC Be and Oe stars using the observed vsini parameters, corrected from the underestimation induced by the gravitational darkening effect. Results: The angular velocities of SMC Oe/Be stars are on average $<Omega/Omega_{rm c}>=0.95$ in the ZAMS. These velocities are in the area theoretically predicted for the LGRBs progenitors. We estimated the yearly rate per galaxy of LGRBs and the number of LGRBs produced in the local Universe up to z=0.2. We have considered that the mass range of LGRB progenitors corresponds to stars hotter than spectral types B0-B1 and used individual beaming angles from 5 to 15degr. We thus obtain $R^{rm pred}_{rm LGRB}sim10^{-7}$ to $sim10^{-6}$ LGRBs/year/galaxy, which represents on average 2 to 14 LGRB predicted events in the local Universe during the past 11 years. The predicted rates could widely surpass the observed ones [(0.2-3)$times10^{-7}$ LGRBs/year/galaxy; 8 LGRBs observed in the local Universe during the last 11 years] if the stellar counts were made from the spectral type B1-B2, in accordance with the expected apparent spectral types of the appropriate massive fast rotators. Conclusion: We conclude that the massive Be/Oe stars with SMC metallicity could be LGRBs progenitors. Nevertheless, other SMC O/B stars without emission lines, which have high enough specific angular momentum, can enhance the predicted $R_{rm LGRB}$ rate.
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.
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