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Register a new userThe CH fraction of Carbon stars at high Galactic latitudes
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CH stars form a distinct class of objects with characteristic properties like iron deficiency, enrichment of carbon and overabundance in heavy elements. These properties can provide strong observational constraints for theoretical computation of nucleosynthesis at low-metallicity. An important question is the relative surface density of CH stars which can provide valuable inputs to our understanding on the role of low to intermediate-mass stars in the early Galactic chemical evolution. Spectroscopic characterization provides an effective way of identifying CH stars. The present analysis is aimed at a quantitative assessment of the fraction of CH stars in a sample of stars using a set of spectral classification criteria. The sample consists of 92 objects selected from a collection of candidate Faint High Latitude Carbon stars from the Hamburg/ESO survey. Medium resolution (R ~ 1300) spectra for these objects were obtained using OMR at VBO, Kavalur and HFOSC at HCT, IAO, Hanle, during 2007 - 2009 spanning a wavelength range 3800 - 6800 A. Spectral analysis shows 36 of the 92 objects to be potential CH stars; combined with our earlier studies (Goswami 2005, Goswami et al. 2007) this implies ~ 37% (of 243) objects as the CH fraction. We present spectral descriptions of the newly identified CH star candidates. Estimated effective temperatures, 12C/13C isotopic ratios and their locations on the two colour J-H vs H-K plot are used to support their identification.
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Very little is known about the polarimetric properties of CH stars and carbon-enhanced metal-poor (CEMP) stars, although many of these objects have been studied in detail both photometrically and spectroscopically. We aim to derive polarimetric properties for a large sample of CEMP stars and CH stars to fill this gap. Multiband polarimetric observations were conducted in the first run for a sample of twenty-nine objects that include twenty-two CEMP and CH stars and seven polarization standards. Estimates of polarization were obtained using standard procedures of polarization calculation. Five objects in our sample do not show any significant polarization over the different colours of BVRI. For the rest of the objects the derived percentage polarization estimates are less than or equal to 1%, and they are found to exhibit random behaviour with respect to the inverse of the effective wavelength of observations. Polarization also does not seem to have any correlation with the effective temperatures of the stars. Our polarimetric estimates indicate there are circumstellar envelopes around these stars that are spherically symmetric or envelopes with little or no dust. In the plane of differential polarization, defined as the difference between the maximum and the minimum polarizations within the BVRI-bands, versus their visual magnitude, the stars appear to be confined to a narrow band. The implication of this trend for understanding the nature of the circumstellar environment remains to be determined and requires detailed modelling.
Motivated by the historical identification of runaway main-sequence (MS) stars of early spectral type at high Galactic latitudes, we test the capability of Gaia at identifying new such stars. We have selected ~2300 sources with Gaia magnitudes of GBP - GRP < 0.05, compatible with the colors of low-extinction MS stars earlier than mid-A spectral type, and obtained low-resolution optical spectroscopy for 48 such stars. By performing detailed photometric and spectroscopic analyses, we derive their atmospheric and physical parameters (effective temperature, surface gravity, radial velocity, interstellar reddening, spectrophotometric distance, mass, radius, luminosity, and age). The comparison between spectrophotometric and parallax-based distances enables us to disentangle the MS candidates from older blue horizontal branch (BHB) candidates. We identify 12 runaway MS candidates, with masses between 2 and 6 Msun. Their trajectories are traced back to the Galactic disc to identify their most recent Galactic plane crossings and the corresponding flight times. All 12 candidates are ejected from the Galactic disc within 2 to 16.5 kpc from the Galactic center and possess flight times that are shorter than their evolutionary ages, compatible with a runaway hypothesis. Three MS candidates have ejection velocities exceeding 450 km/s, thus, appear to challenge the canonical ejection scenarios for late B-type stars. The fastest star of our sample also has a non-negligible Galactic escape probability if its MS nature can be confirmed. We identify 27 BHB candidates, and the two hottest stars in our sample are rare late O and early B type stars of low mass evolving towards the white dwarf cooling sequence.
Polarization is an important indicator of stellar evolution, especially for stars evolving from red-giant stage to planetary nebulae. However, not much is known about the polarimetric properties of the carbon-enhanced metal-poor (CEMP) stars, although they have been well studied in terms of photometric as well as low- and high-resolution spectroscopy. We report here first-ever estimates of V-band polarimetry of a group of CEMP stars. V-band polarimetry was planned as the V-band is known to show maximum polarization among BVRI polarimetry for any scattering of light caused due to dust. Based on these estimates the program stars show a distinct classification into two: one with p% < 0.4 and the other with p% > 1. Stars with circumstellar material exhibit a certain amount of polarization that may be caused by scattering of starlight due to circumstellar dust distribution into non-spherically symmetric envelopes. The degree of polarization increases with asymmetries present in the geometry of the circumstellar dust distribution. Our results reflect upon these properties. While the sample size is relatively small, the polarimetric separation of the two groups (p% < 0.4 and p% > 1) is very distinct; this finding, therefore, opens up an avenue of exploration with regard to CEMP stars.
White-light flares are magnetically driven localized brightenings on the surfaces of stars. Their temporal, spectral, and statistical properties present a treasury of physical information about stellar magnetic fields. The spatial distributions of magnetic spots and associated flaring regions help constrain dynamo theories. Moreover, flares are thought to crucially affect the habitability of exoplanets that orbit these stars. Measuring the location of flares on stars other than the Sun is challenging due to the lack of spatial resolution. Here we present four fully convective stars observed with the Transiting Exoplanet Survey Satellite (TESS) that displayed large, long-duration flares in white-light which were modulated in brightness by the stars fast rotation. This allowed us to determine the loci of these flares directly from the light curves. All four flares occurred at latitudes between 55 deg and 81 deg, far higher than typical solar flare latitudes. Our findings are evidence that strong magnetic fields tend to emerge close to the stellar rotational poles for fully convective stars, and suggest that the impact of flares on the habitability of exoplanets around small stars could be weaker than previously thought.
The extended solar cycle 24 began in 1999 near 70 degrees latitude, similarly to cycle 23 in 1989 and cycle 22 in 1979. The extended cycle is manifested by persistent Fe XIV coronal emission appearing near 70 degrees latitude and slowly migrating towards the equator, merging with the latitudes of sunspots and active regions (the butterfly diagram) after several years. Cycle 24 began its migration at a rate 40% slower than the previous two solar cycles, thus indicating the possibility of a peculiar cycle. However, the onset of the Rush to the Poles of polar crown prominences and their associated coronal emission, which has been a precursor to solar maximum in recent cycles (cf. Altrock 2003), has just been identified in the northern hemisphere. Peculiarly, this Rush is leisurely, at only 50% of the rate in the previous two cycles. The properties of the current Rush to the Poles yields an estimate of 2013 or 2014 for solar maximum.
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