14695100
Nature of two massive protostellar candidates: IRAS 21307+5049 and IRAS 22172+5549
We present observations of continuum and molecular lines towards the protostar candidates IRAS 21307+5049 and IRAS 22172+5549. Single-dish maps in the 12 CO (2–1), C 18 O (2–1), H 13 CO +  (1–0) lines and 850 μ m continuum are compared with interferometric maps in the 12 CO (1–0) line and 3 mm continuum, and with mid- and near-infrared images. Also, single-pointing spectra of the CH 3 C 2 H (6–5), (8–7) and (13–12) lines observed towards IRAS 21307+5049, and H 2 and [FeII] line emission observed towards IRAS 22172+5549, are presented. A plausible interpretation of our data based on the continuum maps and spectral energy distributions is that two components are present: a compact molecular core, responsible for the continuum emission at wavelengths longer than ~$25\;\mu$m, and a cluster of stars located close to the center of the core, but not spatially coincident with it, responsible for the emission at shorter wavelengths. The core is approximately located at the center of the associated molecular outflow, detected for both sources in the 12 CO (1–0) and (2–1) lines. The cores have masses of ~$50\;M_{\odot}$, and luminosities of ~$10^{3}\;L_{\odot}$. The outflows parameters are consistent with those typically found in high-mass young stellar objects.  Our results support the hypothesis that in these sources the luminosity is dominated by accretion rather than by nuclear burning. We conclude that the sources embedded inside the cores are likely protostars with mass ~$ 5{-}8\;M_{\odot}$.
1085808
Long-term study of water masers associated with Young Stellar Objects - II. Analysis
We present the analysis of the properties of water maser emission in 14 star forming regions (SFRs), which have been monitored for up to 13 years with a sampling rate of about once every 2–3 months. The 14 regions were chosen to span a range in luminosity L fir  of the associated Young Stellar Object (YSO) between 20 $L_{\odot}$ and $1.8 \times 10^6$ $L_{\odot}$. The general scope of the analysis is to investigate the dependence of the overall spectral morphology of the maser emission and its variability on the luminosity of the YSO. 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 L fir  $\ga3 \times 10^4$ $L_{\odot}$ the maser emission becomes increasingly structured and more extended in velocity with increasing L fir . Below this limit the maser emission shows the same variety of morphologies, but without a clear dependence on L fir  and with a smaller velocity extent. Also, for sources with L fir  above this limit, the water maser is always present above the 5 σ -level; below it, the typical 5 σ detection rate is 75–80%. Although the present sample contains few objects with low YSO luminosity, we can conclude that there must be a lower limit to L fir  (${\la} 430$ $L_{\odot}$), 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. For individual emission components the presence of velocity gradients seems to be quite common; we find both acceleration and deceleration, with values between 0.02 and 1.8 km s -1  yr -1 . From the 14 “bursts” that we looked at in some detail we derive durations of between 60 and 900 days and flux density increases of between 40% and $\ga$1840% (with an absolute maximum of ∼ 820 Jy over 63 days). The ranges found in burst- intensity and -duration are biased by our minimum sampling interval, while the lifetime of the burst is furthermore affected by the fact that bursts of very long duration may not be recognized as such.  In addition to the flux density variations in individual emission components, the H 2 O maser output as a whole is found to exhibit a periodic long-term variation in several sources. This may be a consequence of periodic variations in the wind/jets from the exciting YSO.
5428392
Search for massive protostellar candidates in the southern hemisphere: I. Association with dense gas
We have observed two rotational transitions of both CS and C 17 O, and the 1.2 mm continuum emission towards a sample of 130 high-mass protostellar candidates with δ δ ≥ $-30^{\circ}$, we have selected from the IRAS Point Source Catalogue 429 sources which potentially are compact molecular clouds on the basis of their IR colours. The sample has then been divided into two groups according to the colour indices [25–12] and [60–12]: the 298 sources with [25–12]  ≥ 0.57 and [60–12]  ≥ 1.30 have been called High sources, the remaining 131 have been called Low sources. In this paper, we check the association with dense gas and dust in 130 Low sources. We have obtained a detection rate of ~$85\%$ in CS, demonstrating a tight association of the sources with dense molecular clumps. Among the sources detected in CS, ~$76\%$ have also been detected in C 17 O  and ~$93\%$ in the 1.2 mm continuum. Millimeter-continuum maps show the presence of clumps with diameters in the range $0.2{-}2$ pc and masses from a few $M_{\odot}$ to $10^{5}~M_{\odot}$; H 2  volume densities computed from CS line ratios lie between ~10 4.5 and 10 5.5  cm -3 . The bolometric luminosities of the sources, derived from IRAS data, are in the range $10^{3}{-}10^{6}~L_{\odot}$, consistent with embedded high-mass objects. Based on our results and those found in the literature for other samples of high-mass young stellar objects, we conclude that our sources are massive objects in a very early evolutionary stage, probably prior to the formation of an Hii region. We propose a scenario in which High and Low sources are both made of a massive clump hosting a high-mass protostellar candidate and a nearby stellar cluster. The difference might be due to the fact that the 12  μ m IRAS flux, the best discriminant between the two groups, is dominated by the emission from the cluster in Lows and from the massive protostellar object in Highs .
18606865
Galactic interstellar 18 O/ 17 O ratios - a radial gradient?
Context. The determination of interstellar abundances is essential for a better understanding of stellar nucleosynthesis and the “chemical” evolution of the Galaxy. Aims. The aim is to determine 18 O/ 17 O abundance ratios across the entire Galaxy. These provide a measure of the amount of enrichment by high-mass versus intermediate-mass stars. Methods. Such ratios, derived from the C 18 O and C 17 O $J = 1$–0 lines alone, may be affected by systematic errors. Therefore, the C 18 O and C 17 O (1–0), (2–1), and (3–2), as well as the 13 CO(1–0) and (2–1) lines, were observed towards 18 prominent galactic targets (a total of 25 positions). The combined dataset was analysed with a large velocity gradient model, accounting for optical depth effects. Results. The data cover galactocentric radii between 0.1 and 16.9 kpc (solar circle at 8.5 kpc). Near the centre of the Galaxy, 18 O/ 17 O = 2.88  ±  0.11. For the galactic disc out to a galactocentric distance of ~10 kpc, 18 O/ 17 O = 4.16  ±  0.09. At ~16.5 kpc from the galactic centre, 18 O/ 17 O = 5.03  ±  0.46. Assuming that 18 O is synthesised predominantly in high-mass stars ($M >8$ $M_{\odot}$), while C 17 O is mainly a product of lower mass stars, the ratio from the inner Galaxy indicates a dominance of CNO-hydrogen burning products that is also apparent in the carbon and nitrogen isotope ratios. The high 18 O/ 17 O value of the solar system (5.5) relative to that of the ambient interstellar medium suggests contamination by nearby high-mass stars during its formation. The outer Galaxy poses a fundamental problem. High values in the metal-poor environment of the outer Galaxy are not matched by the low values observed towards the even more metal-poor Large Magellanic Cloud. Apparently, the outer Galaxy cannot be considered as an intermediate environment between the solar neighbourhood and the interstellar medium of small metal-poor galaxies. The apparent 18 O/ 17 O gradient along the galactic disc and the discrepancy between outer disc and LMC isotope ratios may be explained by different ages of the respective stellar populations. More data from the central and far outer parts of the Galaxy are, however, needed to improve the statistical significance of our results.
16173830
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.
12498400
A molecular-line study of clumps with embedded high-mass protostar candidates
We present molecular line observations made with the IRAM 30-m telecopes 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. In a preceding series of papers we have studied a sample of objects, which on the basis of their IR colours are likely to be associated with compact molecular clouds. The original sample of 260 objects was divided approximately evenly into a High group, with IR colour indices [25-12] ≥ 0.57 and [60-12] ≥ 1.3, and a Low group with complementary colours. The FIR luminosity of the Low sources, their distribution in the IR colour-colour diagram, and their lower detection rate in H_2O maser emission compared to the High sources, led to the hypothesis that the majority of these objects represent an earlier stage in the evolution than the members of the High group, which are mostly identifiable with UC HII regions. Subsequent observations led to the selection of 12 Low sources that have FIR luminosities indicating the presence of B2.5 to O8.5 V0 stars, are associated with dense gas and dust, have (sub-)mm continuum spectra indicating temperatures of ~30 K, and have no detectable radio continuum emission. One of these sources has been proposed by us to be a good candidate for the high-mass equivalent of a Class 0 object. In the present paper we present observations of the molecular environment of 11 of these 12 objects, with the aim to derive the physical parameters of the gas in which they are embedded, and to find further evidence in support of our hypothesis that these sources are the precursors to UC HII regions. We find that the data are consistent with such an interpretation. 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 ~0.5-1 pc (with a range of 0.2 to 2.2 pc), and masses of a few tens to a few thousand solar masses. Compared to a similar analysis of High sources, 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 in which the High sources are embedded. The large sizes suggest these clumps should still undergo substantial contraction (their densities are ~10 times smaller than those of the High sources). The lower temperatures and small linewidths are also expected in objects in an earlier evolutionary state. In various sources indications are found for outflowing gas, though its detection is hampered by the presence of multiple emission components in the line spectra. There are also signs of self-absorption, especially in the spectra of ^(13)CO and HCO+. We find that the masses of the molecular clumps associated with our objects increase with L_(fir) (M_(clump) ∝ L^(1.17)_(fir)), and that there is a (weak) relation between the clump mass and the mass of the embedded protostellar object M_(proto) ∝ M^(0.30)_(clump). The large amount of observational data is necessarily presented in a compact, reduced form. Yet we supply enough information to allow further study. These data alone cannot prove or disprove the hypothesis that among these objects a high-mass protostar is truly present. More observations, at different wavelenghts and spatial resolutions are needed to provide enough constraints on the number of possible interpretations.
120833712
Subterranean ground motion studies for the Einstein Telescope
Seismic motion limits the low-frequency sensitivity of ground-based gravitational wave detectors. A conceptual design study into the feasibility of a future-generation gravitational wave observatory, coined the Einstein Telescope, has been completed. As part of this design phase, we performed a ground motion study to determine the seismic noise characteristics at various sites across the globe. This investigation focused on underground sites and encompassed a variety of geologies, including clay, salt, and hard rock, at 15 locations in nine European countries, the USA, and Japan. In addition, we analyzed data from the Virtual European Broadband Seismograph Network to characterize European seismic motion. We show that, in the region of interest for future-generation gravitational wave detectors (1–10 Hz), seismic motion is dominated by activity from anthropogenic sources. A number of sites were found that exhibited a reduction in seismic power of several orders of magnitude with respect to current detector sites, thus making it possible to set requirements for the Einstein Telescope seismic noise environment.
123641202
Seismic Attenuation System (SAS) in the Kamioka Mine