اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدThe RCB star V854 Cen is surrounded by a hot dusty shell
251
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Aims : The hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars might be the result of a double-degenerate merger of two white dwarfs (WDs), or a final helium shell flash in a planetary nebula central star. In this context, any information on the geometry of their circumstellar environment and, in particular, the potential detection of elongated structures, is of great importance. Methods : We obtained near-IR observations of V854 Cen with the AMBER recombiner located at the Very Large Telescope Interferometer (VLTI) array with the compact array (B$leq$35m) in 2013 and the long array (B$leq$140m) in 2014. At each time, V854 Cen was at maximum light. The $H$- and $K$-band continua were investigated by means of spectrally dependant geometric models. These data were supplemented with mid-IR VISIR/VLT images. Results : A dusty slightly elongated over density is discovered both in the $H$- and $K$-band images. With the compact array, the central star is unresolved ($Thetaleq2.5$,mas), but a flattened dusty environment of $8 times 11$ mas is discovered whose flux increases from about $sim$20% in the $H$ band to reach about $sim$50% at 2.3$micron$, which indicates hot (T$sim$1500,K) dust in the close vicinity of the star. The major axis is oriented at a position angle (P.A.) of 126$pm$29$deg$. Adding the long-array configuration dataset provides tighter constraints on the star diameter ($Thetaleq1.0$ mas), a slight increase of the overdensity to $12 times 15$ mas and a consistent P.A. of 133$pm$49$deg$. The closure phases, sensitive to asymmetries, are null and compatible with a centro-symmetric, unperturbed environment excluding point sources at the level of 3% of the total flux in 2013 and 2014. The VISIR images exhibit a flattened aspect ratio at the 15-20% level at larger distances ($sim$1$arcsec$) with a position angle of 92$pm$19$deg$, marginally consistent with the interferometric observations. Conclusions : This is the first time that a moderately elongated structure has been observed around an RCB star. These observations confirm the numerous suggestions for a bipolar structure proposed for this star in the literature, which were mainly based on polarimetric and spectroscopic observations.
قيم البحث
اقرأ أيضاً
The helium-peculiar star a Cen exhibits line profile variations of elements such as iron, nitrogen and oxygen in addition to its well-known extreme helium variability. New high S/N, high-resolution spectra are used to perform a quantitative measureme
nt of the abundances of the star and determine the relation of the concentrations of the heavier elements on the surface of the star to the helium concentration and the magnetic field orientation. Doppler images have been created using programs described in earlier papers by Rice and others. An alternative surface abundance mapping code has been used to model the helium line variations after our Doppler imaging of certain individual helium lines produced mediocre results. We confirm the long-known existence of helium-rich and helium-poor hemispheres on a Cen and we measure a difference of more than two orders of magnitude in helium abundance from one side of the star to the other. Helium is overabundant by a factor of about 5 over much of the helium-rich hemisphere. Of particular note is our discovery that the helium-poor hemisphere has a very high abundance of helium-3, approximately equal to the helium-4 abundance. a Cen is therefore a new member of the small group of helium-3 stars and the first well-established magnetic member of the class. For the three metals investigated here, there are two strong concentrations of abundance near the equator consistent with the positive magnetic maximum and two somewhat weaker concentrations of abundance where the helium concentration is centered and roughly where the negative peak of the magnetic field would be found. Another strong concentration is found near the equator and this is not explainable in terms of any simple symmetry with the helium abundance or the apparent magnetic field main polar locations.
We report the discovery of a spiral-like nebula with the Wide-field Infrared Survey Explorer (WISE) and the results of optical spectroscopy of its associated star TYC 8606-2025-1 with the Southern African Large Telescope (SALT). We find that TYC 8606
-2025-1 is a G8 III star of $approx3$ M$_odot$, showing a carbon depletion by a factor of two and a nitrogen enhancement by a factor of three. We also derived an excess of s-process elements, most strongly for barium, which is a factor of three overabundant, indicating that TYC 8606-2025-1 is a mild barium star. We thereby add a new member to the small group of barium stars with circumstellar nebulae. Our radial velocity measurements indicate that TYC 8606-2025-1 has an unseen binary companion. The advanced evolutionary stage of TYC 8606-2025-1, together with the presence of a circumstellar nebula, implies an initial mass of the companion of also about 3 M$_odot$. We conclude that the infrared nebula, due to its spiral shape, and because it has no optical counterpart, was ejected by the companion as a consequence of a very late thermal pulse, during about one orbital rotation.
We present spectra of the optical transient of GRB021004 obtained with the Hobby-Eberly telescope starting 15.48, 20.31 hours, and 4.84 days after the burst and a spectrum obtained with the H. J. Smith 2.7 m Telescope starting 14.31 hours after the b
urst. GRB021004 is the first afterglow whose spectrum is dominated by absorption lines from high ionization species with multiple velocity components separated by up to 3000 km/s. We argue that these lines are likely to come from shells around a massive progenitor star. The high velocities and high ionizations arise from a combination of acceleration and flash-ionization by the burst photons and the wind velocity and steady ionization by the progenitor. We also analyze the broad-band spectrum and the light curve. We distinguish six components along the line of sight: (1) The z~2.293 absorption lines arise from the wind of a massive star. For a mass loss rate of ~6 x 10^{-5} solar masses per year, this component also provides the external medium to create the afterglow light. (2) A second shell produces absorption lines with a relative velocity of 560 km/s, and this is associated with the shell created by the fast massive star wind blowing a bubble in the preceding slow wind at a radial distance of order 10 pc. (3) More distant clouds within the host galaxy lie between 30-2500 pc, where they have been ionized by the burst. (4-6) The massive star wind has clumps with radii and over-densities of 0.022, 0.063, and 0.12 parsecs and 50%, 10%, and 10% respectively. The immediate progenitor of the burster could either be a WC-type Wolf-Rayet star or a highly evolved star whose original mass was just too small for it to become a WN-type Wolf-Rayet star.
Abundance analyses of the two newly discovered R Coronae Borealis (RCB) stars ASAS-RCB-8 and ASAS-RCB-10 were conducted using high-resolution optical spectra and model atmospheres. Their chemical compositions place the pair among the majority class o
f RCBs. ASAS-RCB-10 is one of the most N-poor majority RCBs with an above average O abundance. Relative to ASAS-RCB-10, ASAS-RCB-8 is H poor by 1.6 dex, O-poor by 0.7 dex but N-rich by 0.8 dex suggesting a higher contamination by CNO-cycled material.
We consider a small sample of oxygen-rich, asymptotic giant branch stars in the Large Magellanic Cloud, observed by the Spitzer Space Telescope, exhibiting a peculiar spectral energy distribution, which can be hardly explained by the common assumptio
n that dust around AGB stars is primarily composed of silicate grains. We suggest that this uncommon class of objects are the progeny of a metal-poor generation of stars, with metallicity $Z sim 1-2times 10^{-3}$, formed $sim 100$ Myr ago. The main dust component in the circumstellar envelope is solid iron. In these stars the poor formation of silicates is set by the strong nucleosynthesis experienced at the base of the envelope, which provokes a scarcity of magnesium atoms and water molecules, required to the silicate formation. The importance of the present results to interpret the data from the incoming James Webb Space Telescope is also discussed.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
