No Arabic abstract
Context. One of the keys to understanding the origin of the Ap stars and their significance in the general context of stellar astrophysics is the consideration of the most extreme properties displayed by some of them. In that context, HD 965 is particularly interesting, as it combines some of the most pronounced chemical peculiarities with one of the longest rotation periods known. Aims. We characterise the variations of the magnetic field of the Ap star HD 965 and derive constraints about its structure. Methods. We combine published measurements of the mean longitudinal field <Bz> of HD 965 with new determinations of this field moment from circular spectropolarimetry obtained at the 6-m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences. For the mean magnetic field modulus <B>, literature data are complemented by the analysis of ESO archive spectra. Results. We present the first determination of the rotation period of HD 965, P = (16.5+/-0.5) y. HD 965 is only the third Ap star with a period longer than 10 years for which magnetic field measurements have been obtained over more than a full cycle. The variation curve of <Bz> is well approximated by a cosine wave. <B> does not show any significant variation. The observed behaviour of these field moments is well represented by a simple model consisting of the superposition of collinear dipole, quadrupole and octupole. The distribution of neodymium over the surface of HD 965 is highly non-uniform. The element appears concentrated around the magnetic poles, especially the negative one. Conclusions. The shape of the longitudinal magnetic variation curve of HD 965 indicates that its magnetic field is essentially symmetric about an axis passing through the centre of the star. Overall, as far as its magnetic field is concerned, HD 965 appears similar to the bulk of the long-period Ap stars.
We present here results of an optical spectroscopic study of a new Cataclysmic Variable SDSS J001856.93+345444.3. We demonstrate that the most probable value of the orbital period of the system is Porb = 0.6051 pm 0.022 days (=14.5226 hours), based on the measurements of radial velocity of a complex of absorption features emanating from the K2-K4V type secondary component. However, the radial velocity measurements from the emission lines are best folded with the period Pem = 0.5743day (=13.78 hours). The gamma-velocity of the emission lines varies significantly from epoch to epoch. There is an underlying broader and weaker component to the emission lines, which we could not resolve. Based on the appearance of the emission lines, the presence of very strong He II lines and the moderate polarization detected by Dillon et al. (2008), we conclude that SDSS J0018+3454 is an asynchronous magnetic CV (Polar).
The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium1, almost all other elements were created in stars and supernovae. The mass fraction, Z, of elements more massive than helium, is called metallicity. A number of very metal poor stars have been found some of which, while having a low iron abundance, are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with metallicities lower than Z=1.5E-5, it has been suggested that low mass stars (M<0.8Modot, the ones that survive to the present day) cannot form until the interstellar medium has been enriched above a critical value, estimated to lie in the range 1.5E-8leqZleq1.5E-6, although competing theories claiming the contrary do exist. Here we report the chemical composition of a star with a very low Zleq6.9E-7 (4.5E-5 of that of the Sun) and a chemical pattern typical of classical extremely metal poor stars, meaning without the enrichment of carbon, nitrogen and oxygen. This shows that low mass stars can be formed at very low metallicity. Lithium is not detected, suggesting a low metallicity extension of the previously observed trend in lithium depletion. Lithium depletion implies that the stellar material must have experienced temperatures above two million K in its history, which points to rather particular formation condition or internal mixing process, for low Z stars.
The high velocity dispersion compact cloud CO-0.30-0.07 is a peculiar molecular clump discovered in the central moleculr zone of the Milky Way, which is characterized by its extremely broad velocity emissions ($sim 145 rm{km s^{-1}}$) despite the absence of internal energy sources. We present new interferometric maps of the cloud in multiple molecular lines in frequency ranges of 265--269 GHz and 276--280 GHz obtained using the Sumbmillimeter Array, along with the single-dish images previously obtained with the ASTE 10-m telescope. The data show that the characteristic broad velocity emissions are predominantly confined in two parallel ridges running through the cloud center. The central ridges are tightly anti-correlated with each other in both space and velocity, thereby sharply dividing the entire cloud into two distinct velocity components (+15 km s$^{-1}$ and +55 km s$^{-1}$). This morphology is consistent with a model in which the two velocity components collide with a relative velocity of 40 $mathrm{km s^{-1}}$ at the interface defined by the central ridges, although an alternative explanation with a highly inclined expanding-ring model is yet to be fully invalidated. We have also unexpectedly detected several compact clumps ($lesssim 0.1 $pc in radius) likely formed by shock compression. The clumps have several features in common with typical star-forming clouds: high densities ($10^{6.5-7.5} mathrm{cm^{-3}}$), rich abundances of hot-core-type molecular species, and relatively narrow velocity widths apparently decoupled from the furious turbulence dominating the cloud. The cloud CO-0.30-0.07 is possibly at an early phase of star formation activity triggered by the shock impact.
We focus here on one particular and poorly studied object, IRAS11472-0800. It is a highly evolved post-Asymptotic Giant Branch (post-AGB) star of spectral type F, with a large infrared excess produced by thermal emission of circumstellar dust. We deploy a multi-wavelength study which includes the analyses of optical and IR spectra as well as a variability study based on photometric and spectroscopic time-series. The spectral energy distribution (SED) properties as well as the highly processed silicate N-band emission show that the dust in IRAS11472-0800 is likely trapped in a stable disc. The energetics of the SED and the colour variability show that our viewing angle is close to edge-on and that the optical flux is dominated by scattered light. With photospheric abundances of [Fe/H] = -2.7 and [Sc/H]=-4.2, we discovered that IRAS11472-0800 is one of the most chemically-depleted objects known to date. Moreover, IRAS11472-0800 is a pulsating star with a period of 31.16 days and a peak-to-peak amplitude of 0.6 mag in V. The radial velocity variability is strongly influenced by the pulsations, but the significant cycle-to-cycle variability is systematic on a longer time scale, which we interpret as evidence for binary motion. We conclude that IRAS11472-0800 is a pulsating binary star surrounded by a circumbinary disc. The line-of-sight towards the object lies close the the orbital plane making that the optical light is dominated by scattered light. IRAS11472-0800 is one of the most chemically-depleted objects known so far and links the dusty RV,Tauri stars to the non-pulsating class of strongly depleted objects.
We present a catalog of 417 luminous infrared variable stars with periods exceeding 250 days. These were identified in 20 nearby galaxies by the ongoing SPIRITS survey with the Spitzer Space Telescope. Of these, 359 variables have $M_{[4.5]}$ (phase-weighted mean magnitudes) fainter than $-12$ and periods and luminosities consistent with previously reported variables in the Large Magellanic Cloud. However, 58 variables are more luminous than $M_{[4.5]} = -12$, including 11 that are brighter than $M_{[4.5]} = -13$ with the brightest having $M_{[4.5]} = -15.51$. Most of these bright variable sources have quasi-periods longer than 1000 days, including four over 2000 days. We suggest that the fundamental period-luminosity relationship, previously measured for the Large Magellanic Cloud, extends to much higher luminosities and longer periods in this large galaxy sample. We posit that these variables include massive AGB stars (possibly super-AGB stars), red supergiants experiencing exceptionally high mass-loss rates, and interacting binaries. We also present 3.6, 4.5, 5.8 and 8.0 $mu$m photometric catalogs for all sources in these 20 galaxies.