HotMobile 2008: Postconference Report

HotMobile 2008 presented a two-day program on mobile computing systems and applications. The authors focuses on the sessions on sensors, modularity, wireless, security, systems, and screens. The mobile device is the most amazing invention in history and that it has had the largest impact on human kind. Because mobile phones combine mobile devices with ongoing developments in software and communication technologies, they have the potential to change the way people think and act

CO in OH/IR stars close to the Galactic centre

Aims: A pilot project has been carried out to measure circumstellar CO emission from three OH/IR stars close to the Galactic centre. The intention was to find out whether it would be possible to conduct a large-scale survey for mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA). Such a survey would increase our understanding of the evolution of the Galactic bulge. Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is necessary as a `spatial filter' in this region of space because of the confusion with interstellar CO emission. Results: Towards two of the stars, CO emission was detected with positions and radial velocities coinciding within the statistical errors with the corresponding data of the associated OH sources. However, for one of the stars the line profile is not what one expects for an unresolved expanding circumstellar envelope. We believe that this CO envelope is partially resolved and that this star therefore is a foreground star not belonging to the bulge. Conclusions: The results of the observations have shown that it is possible to detect line profiles of circumstellar CO from late-type stars both within and in the direction of the Galactic bulge. ALMA will be able to detect CO emission in short integrations with sensitivity sufficient to estimate mass-loss rates from a large number of such stars.Comment: 5 pages, 3 figure

Solving the polarization problem in ALMA-VLBI observations

The Atacama Large mm-submm Array (ALMA) is, by far, the most sensitive mm/submm telescope in the World. The ALMA Phasing Project (APP) will allow us to phase-up all the ALMA antennas and use them as one single VLBI station. This will be a key component of the Event Horizon Telescope (EHT), a Global VLBI array at millimeter wavelengths. A problem in the APP is the calibration and conversion of the polarization channels. Most VLBI stations record their signals in a circular basis, but the ALMA receivers record in a linear basis. The strategy that will be followed in the phased-ALMA VLBI observations will be to correlate in "mixed" basis (i.e., linear versus circular) and convert the visibilities to a pure circular basis after the correlation. We have developed an algorithm to perform such a polarization conversion of the VLBI visibilities. In these proceedings, we present the basics of the PolConvert algorithm and discuss on the polarization conversion in the general case were single dishes (besides phased arrays) record with linear receivers in VLBI observations. We show some results of PolConvert applied to realistic simulations, as well as a test with real VLBI observations at 86\,GHz between the Onsala radiotelescope (recording in linear basis) and the Effelsberg radiotelescope (recording in circular basis).Comment: To appear in the Proceedings of the 12th European VLBI Network Symposium (7-10 Oct 2014, Cagliary, Italy

Preliminary results on SiO v=3 J=1-0 maser emission from AGB stars

We present the results of SiO maser observations at 43GHz toward two AGB stars using the VLBA. Our preliminary results on the relative positions of the different J=1-0 SiO masers (v=1,2 and 3) indicate that the current ideas on SiO maser pumping could be wrong at some fundamental level. A deep revision of the SiO pumping models could be necessary.Comment: poster, 2 pages, 2 figures, Proc. IAU Symp. 287 "Cosmic Masers: from OH to H0", R.S. Booth, E.M.L. Humphreys and W.H.T. Vlemmings, ed

SiO masers from AGB stars in the vibrationally excited v=1,v=2, and v=3 states

The v=1 and v=2 J=1-0 (43 GHz), and v=1 J=2-1 (86 GHz) SiO masers are intense in AGB stars and have been mapped using VLBI showing ring-like distributions. Those of the v=1, v=2 J=1-0 masers are similar, but the spots are rarely coincident, while the v=1 J=2-1 maser arises from a well separated region farther out. These relative locations can be explained by models tools that include the overlap of two IR lines of SiO and H2O. The v=3 J=1-0 line is not directly affected by any line overlap and its spot structure and position, relative to the other lines, is a good test to the standard pumping models. We present single-dish and simultaneous VLBI observations of the v=1, v=2, and v=3 J=1-0 maser transitions of 28SiO in several AGB stars. The spatial distribution of the SiO maser emission in the v=3 J=1-0 transition from AGB stars is systematically composed of a series of spots that occupy a ring-like structure. The overall ring structure is extremely similar to that found in the other 43 GHz transitions and is very different from the structure of the v=1 J=2-1 maser. The positions of the individual spots of the different 43 GHz lines are, however, very rarely coincident, which in general is separated by about 0.3 AU (between 1 and 5 mas). These results are very difficult to reconcile with standard pumping models, which predict that the masers of rotational transitions within a given vibrational state require very similar excitation conditions, while the transitions of different vibrational states should appear in different positions. However, models including line overlap tend to predict v=1, v=2, v=3 J=1-0 population inversion to occur under very similar conditions, while the requirements for v=1 J=2-1 appear clearly different, and are compatible with the observational results.Comment: 9 pages, 4 figures accepted by A&

Approximation properties of the qq-sine bases

For $q>12/11$ the eigenfunctions of the non-linear eigenvalue problem associated to the one-dimensional $q$-Laplacian are known to form a Riesz basis of $L^2(0,1)$. We examine in this paper the approximation properties of this family of functions and its dual, in order to establish non-orthogonal spectral methods for the $p$-Poisson boundary value problem and its corresponding parabolic time evolution initial value problem. The principal objective of our analysis is the determination of optimal values of $q$ for which the best approximation is achieved for a given $p$ problem.Comment: 20 pages, 11 figures and 2 tables. We have fixed a number of typos and added references. Changed the title to better reflect the conten

The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types

A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of AGB stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models. In order to constrain the circumstellar HCN abundance distribution a detailed non-LTE excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included. The median values for the derived abundances of HCN (with respect to H2) are 3x10-5, 7x10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available. We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars.Comment: Accepted for publication in A&

The proper motion of the Arches cluster with Keck Laser-Guide Star Adaptive Optics

We present the first measurement of the proper motion of the young, compact Arches cluster near the Galactic center from near-infrared adaptive optics (AO) data taken with the recently commissioned laser-guide star (LGS) at the Keck 10-m telescope. The excellent astrometric accuracy achieved with LGS-AO provides the basis for a detailed comparison with VLT/NAOS-CONICA data taken 4.3 years earlier. Over the 4.3 year baseline, a spatial displacement of the Arches cluster with respect to the field population is measured to be 24.0 +/- 2.2 mas, corresponding to a proper motion of 5.6 +/- 0.5 mas/yr or 212 +/- 29 km/s at a distance of 8 kpc. In combination with the known line-of-sight velocity of the cluster, we derive a 3D space motion of 232 +/- 30 km/s of the Arches relative to the field. The large proper motion of the Arches cannot be explained with any of the closed orbital families observed in gas clouds in the bar potential of the inner Galaxy, but would be consistent with the Arches being on a transitional trajectory from x1 to x2 orbits. We investigate a cloud-cloud collision as the possible origin for the Arches cluster. The integration of the cluster orbit in the potential of the inner Galaxy suggests that the cluster passes within 10 pc of the supermassive black hole only if its true GC distance is very close to its projected distance. A contribution of young stars from the Arches cluster to the young stellar population in the inner few parsecs of the GC thus appears increasingly unlikely. The measurement of the 3D velocity and orbital analysis provides the first observational evidence that Arches-like clusters do not spiral into the GC. This confirms that no progenitor clusters to the nuclear cluster are observed at the present epoch.Comment: 22 pdflatex pages including 12 figures, reviewed version accepted by Ap

The abundance of SiS in circumstellar envelopes around AGB stars

New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including the effect of thermal dust grains in the excitation analysis. We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical models. The carbon stars (C/O>1) have a mean fractional abundance of 3.1E-6, about an order of magnitude higher than found for the M-type AGB stars (C/O<1) where the mean value is 2.7E-7. These numbers are in reasonable agreement with photospheric LTE chemical models. SiS appears to behave similar to SiO in terms of photodissociation in the outer part of the circumstellar envelope. In contrast to previous results for the related molecule SiO, there is no strong correlation of the fractional abundance with density in the CSE, as would be the case if freeze-out onto dust grains were important. However, possible time-variability of the line emission in the lower J transitions and the sensitivity of the line emission to abundance gradients in the inner part of the CSE may mask a correlation with the density of the wind. There are indications that the SiS fractional abundance could be significantly higher closer to the star which, at least in the case of M-type AGB stars, would require non-equilibrium chemical processes.Comment: Accepted for publication in A&A (14 pages, 7 figures