arXiv subjects

J. Brand

Publications and source records attributed to J. Brand.

81 records · Page 5Linked to original sources

arXiv2003

IRAS 23385+6053: a candidate protostellar massive object

We present the results of a multi-line and continuum study towards the source IRAS 23385+6053,performed with the IRAM-30m telescope, the Plateau de Bure Interferometer, the Very Large Array Interferometer and the James Clerk Maxwell Telescope. The new results confirm our earlier findings, namely that IRAS 23385+6053 is a good candidate high-mass protostellar object, precursor of an ultracompact H$_{II}$ region. The source is roughly composed of two regions: a molecular core $\sim0.03\div0.04$ pc in size, with a temperature of $\sim40$ K and an H$_{2}$ volume density of the order of 10$^{7}$ cm$^{-3}$, and an extended halo of diameter $\leq$0.4 pc, with an average kinetic temperature of $\sim 15$ K and H$_{2}$ volume density of the order of 10$^{5}$ cm$^{-3}$. The core temperature is much smaller than what is typically found in molecular cores of the same diameter surrounding massive ZAMS stars. We deduce that the core luminosity is between 150 and $1.6\times10^{4}L_{\odot}$, and we believe that the upper limit is near the ``true'' source luminosity. Moreover, by comparing the H$_{2}$ volume density obtained at different radii from the IRAS source, we find that the halo has a density profile of the type $n_{\rm H_{2}}\propto r^{-2.3}$. This suggests that the source is gravitationally unstable. Finally, we demonstrate that the temperature at the core surface is consistent with a core luminosity of $10^3 L_{\odot}$ and conclude that we might be observing a protostar still accreting material from its parental cloud, whose mass at present is $\sim 6 M_{\odot}$.

astro-ph↗

arXiv2003

Long-term study of water masers associated with Young Stellar Objects. II: Analysis

(Abridged) H2O masers in 14 SFRs have been monitored once every 2-3 months for up to 13 years. We investigate the dependence of the overall spectral morphology of the maser emission and its variability on the luminosity of the YSO, and look for bursts and gradients in individual components. We find that higher-luminosity sources tend to be associated with stronger and more stable masers.Higher-luminosity YSOs can excite more emission components over a larger range in velocity, yet the emission that dominates the spectra is at a velocity very near that of the molecular cloud in which the objects are embedded. For Lfir > 3E+04 Lo the maser emission becomes increasingly structured and more extended in velocity with increasing Lfir. Below this limit the maser emission shows the same variety of morphologies, but without a clear dependence on Lfir and with a smaller velocity extent. Also, for sources with Lfir above this limit, the water maser is always present above the 5sigma-level; below it, the typical 5sigma detection rate is 75-80%. We conclude that there must be a lower limit to Lfir, below which the associated maser is below the detection level most of the time. These results can be understood in terms of scaled versions of similar SFRs with different YSO luminosities, each with many potential sites of maser amplification, which can be excited provided there is sufficient energy to pump them, i.e. the basic pumping process is identical regardless of the YSO luminosity. In SFRs with lower input energies, the conditions of maser amplification are much closer to the threshold conditions, and consequently more unstable. We find indications that the properties of the maser emission may be determined also by the geometry of the SFR, specifically by the beaming and collimation properties of the outflow driven by the YSO.

astro-ph↗

arXiv2003

A Levinson theorem for scattering from a Bose-Einstein condensate

A relation between the number of bound collective excitations of an atomic Bose-Einstein condensate and the phase shift of elastically scattered atoms is derived. Within the Bogoliubov model of a weakly interacting Bose gas this relation is exact and generalises Levinson's theorem. Specific features of the Bogoliubov model such as complex-energy and continuum bound states are discussed and a numerical example is given.

cond-mat.soft↗

arXiv2002

Single-dish monitoring of circumstellar water masers

We present an overview of the long-term water maser monitoring program of a sample of late-type stars, carried out with the Medicina 32-m and Effelsberg 100-m telescopes, and describe the results in some detail. The role the SRT (Sardinia Radio Telescope) could play in this program is outlined.

astro-ph↗

arXiv2002

A Steeper Stellar Mass Spectrum in the Outer Galaxy?

We discuss the results of high-resolution (~0.1-0.2 pc) BIMA CO observations of the central regions of 3 molecular clouds in the far-outer Galaxy (FOG). We identify clumps and investigate their stability by using the virial theorem, including terms due to gravity, turbulence, magnetic field, and interclump gas pressure, and make a comparison with clumps in local clouds (RMC and Orion B South). While a reasonable combination of these forces can render most clumps stable, an interesting difference between FOG and local clumps emerges when comparing only gravity and turbulence. In the FOG these forces are in equilibrium (virial parameter alpha ~ 1) for clumps down to the lowest masses found (a few Msol), but for local clumps alpha ~ 1 only for clumps with masses larger than a few tens of Msol. Thus it appears that in the FOG gravity is the dominant force down to a much lower mass than in local clouds, implying that gravitational collapse and star formation may occur more readily even in the smallest clumps. This might explain the apparently steeper IMF found in the outer Galaxy.

astro-ph↗

arXiv2001

Clumpy outer Galaxy molecular clouds and the steepening of the IMF

We report the results of high-resolution (~0.2 pc) CO(1-0) and CS(2-1) observations of the central regions of three star-forming molecular clouds in the far-outer Galaxy (~16 kpc from the Galactic Center): WB89 85 (Sh 2-127), WB89 380, and WB89 437. We used the BIMA array in combination with IRAM 30-m and NRAO 12-m observations. The GMC's in which the regions are embedded were studied by means of KOSMA 3-m CO(2-1) observations. The properties the CO and CS clumps are analyzed and compared with newly derived results of previously published single-dish measurements of local clouds (OrionB South and Rosette). We find that the slopes of the clump mass distributions (-1.28 and -1.49, for WB89 85 and WB89 380, respectively) are somewhat less steep than found for most local clouds, but similar to those of clouds which have been analyzed with the same clumpfind program. We investigate the clump stability by using the virial theorem, including all possible contributions (gravity, turbulence, magnetic fields, and pressure due to the interclump gas). It appears that under reasonable assumptions a combination of these forces would render most clumps stable. Comparing only gravity and turbulence, we find that in the far-outer Galaxy clouds, these forces are in equilibium (virial parameter alpha~1) for clumps down to the lowest masses found (a few Msol). For clumps in the local clouds alpha~1 only for clumps with masses larger than a few tens of Msol. Thus it appears that in these outer Galaxy clumps gravity is the dominant force down to a much lower mass than in local clouds, implying that gravitational collapse and star formation may occur more readily even in the smallest clumps. Although there are some caveats, due to the inhomogeneity of the data used, this might explain the apparently steeper IMF found in the outer Galaxy.

astro-ph↗

arXiv2001

A study of the H2O maser emission from R Cas

We describe the monitoring program we have been carrying out during the past 10 years on a sample of late-type stars, with the Medicina 32-m and Effelsberg 100-m antennas. We discuss the observations on the Mira-variable R Cas in some detail.

astro-ph↗

arXiv2001

A molecular-line study of clumps with embedded high-mass protostar candidates

We present molecular line observations made with the IRAM 30-m telescope of the immediate surroundings of a sample of 11 candidate high-mass protostars. These observations are part of an effort to clarify the evolutionary status of a set of objects which we consider to be precursors of UC HII regions. We find that the present data are consistent with such an interpretation. The sources have FIR luminosities indicating the presence of B2.5 to O8.5V0 stars, are associated with dense gas and dust, have (sub-)mm continuum spectra indicating temperatures of ca. 30K, and have no detectable radio continuum emission. All observed sources are associated with well-defined molecular clumps. Masses, sizes, and other parameters depend on the tracer used, but typically the cores have average diameters of ca. 0.5-1 pc, and masses of a few tens to a few thousand solar masses. Compared to a similar analysis of sources which are mostly identifyable with UC HII regions, the present sample has molecular clumps that are more massive, larger, cooler, and less turbulent. They also tend to have a smaller ratio of virial-to-luminous mass, indicating they are less dynamically stable than their counterparts embedded in the UC HII-like sources. The large sizes suggest these clumps should still undergo substantial contraction. The lower temperatures and small linewidths are also expected in objects in an earlier evolutionary state. We find that the masses of the molecular clumps associated with our objects increase with Lfir [M(clump) propto Lfir^(1.17)], and that there is a (weak) relation between the clump mass and the mass of the embedded protostellar object M(proto) propto M(clump)^(0.30).

astro-ph↗

arXiv2000

Dark soliton creation in Bose-Einstein condensates

It is demonstrated that stable, standing dark solitons can be created in current dilute-gas Bose-Einstein condensate experiments by the proper combination of phase and density engineering. Other combinations result in a widely controllable range of grey solitons. The phonon contribution is small and is calculated precisely. The ensuing dynamics should be observable in situ, i.e. without ballistic expansion of the condensate.

cond-mat↗