Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDiscovery of Reflection Nebulosity Around Five Vega-like Stars
80
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Coronagraphic optical observations of six Vega-like stars reveal reflection nebulosities, five of which were previously unknown. The nebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble that of the Pleiades, indicating an interstellar origin for dust grains. The reflection nebulosity around HD 123160 has a double-arm morphology, but no disk-like feature is seen as close as 2.5 arcsec from the star in K-band adaptive optics data. We demonstrate that uniform density dust clouds surrounding HD 23362, HD 23680 and HD 123160 can account for the observed 12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD 49662 an additional emission source, such as from a circumstellar disk or non-equilibrium grain heating, is required to fit the 12-25 micron data. These results indicate that in some cases, particularly for Vega-like stars located beyond the Local Bubble (>100 pc), the dust responsible for excess thermal emission may originate from the interstellar medium rather than from a planetary debris system.
rate research
Read More
Aims: To determine the metallicities of 113 Southern Hemisphere Vega-like candidate stars in relation to the Exoplanet host group and field stars. Methods: We applied two spectroscopic methods of abundance determinations: equivalent width measurements together with the ATLAS9 (Kurucz 1993) model atmospheres and the WIDTH9 program, and a comparison of observed spectra with the grid of synthetic spectra of Munari et al. (2005). Results: For the Vega-like group, the metallicities are indistinguishable from those of field stars not known to be associated with planets or disks. This result is quite different from the metallicities of Exoplanet host stars which are metal-rich in comparison to field stars.
We describe results from a survey for J=3-2 12CO emission from visible stars with an infrared excess. The line is clearly detected in 21 objects, with molecular gas (>10^-3 Jupiter masses) common in targets with infrared excesses >0.01 (>56% of objects). Such high excesses indicate the presence of a disc of opening angle >12 degrees; within this, the optically thick disc prevents CO photodissociation. Two or three stars with associated CO have an excess <0.01, implying a disc opening angle <1 degree. Most line profiles are double-peaked or relatively broad. Model fits, assuming a Keplerian disc, indicate outer radii, R_out, of ~20-300 au. As many as 5 discs have outer radii smaller than the Solar System (50 au), and a further 4 have gas at radii <20 au. R_out is independent of the stellar spectral type (from K through to B9), but is correlated with total dust mass. R_out appears to decrease with time: discs around stars of age 3-7 Myr have a mean radius of ~210 au, whereas discs of age 7-20 Myr are a factor of 3 smaller. The only bona fide debris disc with detected CO is HD9672; this has a double peaked line profile and is the most compact gas disc observed, with a modelled radius 17 au). A fit to HD141569 suggests the gas lies in two rings of radii 90 and 250 au, similar to the scattered light structure. In both AB Aur and HD163296 the sizes of the molecular and dust scattering discs are also similar, suggesting that the gas and small dust grains are co-located.
We present images of the Vega system obtained with the IRAM Plateau de Bure interferometer at 1.3 millimeters wavelength with sub-mJy sensitivity and $sim2farcs5$ resolution (about 20 AU). These observations clearly detect the stellar photosphere and two dust emission peaks offset from the star by $9farcs5$ and $8farcs0$ to the northeast and southwest, respectively. These offset emission peaks are consistent with the barely resolved structure visible in previous submillimeter images, and they account for a large fraction of the dust emission. The presence of two dust concentrations at the observed locations is plausibly explained by the dynamical influence of an unseen planet of a few Jupiter masses in a highly eccentric orbit that traps dust in principle mean motion resonances.
We have observed the 8-13 $mu$m spectrum (R$sim$250) of the Vega-like star candidate HD145263 using Subaru/COMICS. The spectrum of HD145263 shows the broad trapezoidal silicate feature with the shoulders at 9.3 $mu$m and 11.44 $mu$m, indicating the presence of crystalline silicate grains. This detection implies that crystalline silicate may also be commonly present around Vega-like stars. The 11.44 $mu$m feature is slightly shifted to a longer wavelength compared to the usual 11.2-3 $mu$m crystalline forsterite feature detected toward Herbig Ae/Be stars and T Tauri stars. Although the peak shift due to the effects of the grain size can not be ruled out, we suggest that Fe-bearing crystalline olivine explains the observed peak wavelength fairly well. Fe-bearing silicates are commonly found in meteorites and most interplanetary dust particles, which originate from planetesimal-like asteroids. According to studies of meteorites, Fe-bearing silicate must have been formed in asteroidal planetesimals, supporting the scenario that dust grains around Vega-like stars are of planetesimal origin, if the observed 11.44 $mu$m peak is due to Fe-bearing silicates.
We report on five binary pulsars discovered in the Parkes multibeam Galactic plane survey. All of the pulsars are old, with characteristic ages 1-11 Gyr, and have relatively small inferred magnetic fields, 5-90e8 G. The orbital periods range from 1.3 to 15 days. As a group these objects differ from the usual low-mass binary pulsars (LMBPs): their spin periods of 9-88 ms are relatively long; their companion masses, 0.2-1.1 Msun, are, in at least some cases, suggestive of CO or more massive white dwarfs; and some of the orbital eccentricities, 1e-5 < e < 0.002, are unexpectedly large. We argue that these observed characteristics reflect binary evolution that is significantly different from that of LMBPs. We also note that intermediate-mass binary pulsars apparently have a smaller scale-height than LMBPs.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
