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Authors
Roberto Nesci
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We report the spectroscopic and photometric monitoring of a sample of 530 candidate AGB stars in a 5 degrees field, selected from the IPHAS catalog: historic light curves were derived from Asiago IR plates taken in the years 1965 -1984. We found 10 Miras, 5 stars with long term trends, 3 semiregular and 3 irregular. Spectral types from CCD slit spectra gave 8 M-type, 7 C-type and 6 S-type stars. In the color-color plots made from IPHAS and 2MASS catalogs, the S-type and M-type stars occupy the same regions, while C-type stars are well separated. All C-type stars with IR excess showed long term trends in their light curve. Distances of the Mira stars, estimated from their periods and K magnitudes, gave a median value of 4.9 kpc with a large spread. A comparison with astrometric parallaxes from Gaia DR2 is briefly discussed.
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gamma-Cas stars constitute a subgroup of Be stars showing unusually hard and bright X-ray emission. In search for additional peculiarities, we analyzed the TESS lightcurves of 15 gamma-Cas analogs. Their periodograms display broad frequency groups and/or narrow isolated peaks, often superimposed over red noise. The detected signals appear at low frequencies, with few cases of significant signals beyond 5/d (and all of them are faint). The signal amplitudes, and sometimes the frequency content, change with time, even in the absence of outburst events. On the basis of their optical photometric variability, gamma-Cas stars reveal no distinctive behaviour and thus appear similar to Be stars in general.
We report here on the discovery of an extended far-infrared shell around the AGB star, R Cassiopeia, made by AKARI and Spitzer. The extended, cold circumstellar shell of R Cas spans nearly 3 arcmin and is probably shaped by interaction with the interstellar medium. This report is one of several studies of well-resolved mass loss histories of AGB stars under AKARI and Spitzer observing programs labeled Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES).
The gamma-Cas category is a subgroup of Be stars displaying a strong, hard, and variable thermal X-ray emission. An XMM-Newton observation of pi Aqr reveals spectral and temporal characteristics that clearly make this Be star another member of the gamma-Cas category. Furthermore, pi Aqr is a binary but, contrary to gamma-Cas, the nature of the companion to the Be star is known; it is a non-degenerate (stellar) object and its small separation from the Be star does not leave much room for a putative compact object close to the Be disk. This renders the accretion scenario difficult to apply in this system, and, hence, this discovery favors a disk-related origin for the gamma-Cas phenomenon.
We selected a sample of post-AGB candidates in the Magellanic Clouds on the basis of their near- and mid-infrared colour characteristics. Fifteen of the most optically bright post-AGB candidates were observed with the South African Large Telescope in order to determine their stellar parameters and thus to validate or discriminate their nature as post-AGB objects in the Magellanic Clouds. The spectral types of absorption-line objects were estimated according to the MK classification, and effective temperatures were obtained by means of stellar atmosphere modelling. Emission-line objects were classified on the basis of the fluxes of the emission lines and the presence of the continuum. Out of 15 observed objects, only 4 appear to be genuine post-AGB stars (27%). In the SMC, 1 out of 4 is post-AGB, and in the LMC, 3 out 11 are post-AGB objects. Thus, we can conclude that the selected region in the colour-colour diagram, while selecting the genuine post-AGB objects, overlaps severely with other types of objects, in particular young stellar objects and planetary nebulae. Additional classification criteria are required to distinguish between post-AGB stars and other types of objects. In particular, photometry at far-IR wavelengths would greatly assist in distinguishing young stellar objects from evolved ones. On the other hand, we showed that the low-resolution optical spectra appear to be sufficient to determine whether the candidates are post-AGB objects.
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.
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