arXiv subjects

Sergio Molinari

Publications and source records attributed to Sergio Molinari.

65 records · Page 4Linked to original sources

arXiv2011

Multi-line spectral imaging of dense cores in the Lupus molecular cloud

The molecular clouds Lupus 1, 3 and 4 were mapped with the Mopra telescope at 3 and 12 mm. Emission lines from high density molecular tracers were detected, i.e. NH$_3$ (1,1), NH$_3$ (2,2), N$_2$H$^+$ (1-0), HC$_3$N (3-2), HC$_3$N (10-9), CS (2-1), CH$_3$OH (2$_0-1_0$)A$^+$ and CH$_3$OH (2$_{-1}-1_{-1}$)E. Velocity gradients of more than 1 km s$^{-1}$ are present in Lupus 1 and 3 and multiple gas components are present in these clouds along some lines of sight. Lupus 1 is the cloud richest in high density cores, 8 cores were detected in it, 5 cores were detected in Lupus 3 and only 2 in Lupus 4. The intensity of the three species HC$_3$N, NH$_3$ and N$_2$H$^+$ changes significantly in the various cores: cores that are brighter in HC$_3$N are fainter or undetected in NH$_3$ and N$_2$H$^+$ and vice versa. We found that the column density ratios HC$_3$N/N$_2$H$^+$ and HC$_3$N/NH$_3$ change by one order of magnitude between the cores, indicating that also the chemical abundance of these species is different. The time dependent chemical code that we used to model our cores shows that the HC$_3$N/N$_2$H$^+$ and HC$_3$N/NH$_3$ ratios decrease with time therefore the observed column density of these species can be used as an indicator of the chemical evolution of dense cores. On this base we classified 5 out of 8 cores in Lupus 1 and 1 out of 5 cores in Lupus 3 as very young protostars or prestellar cores. Comparing the millimetre cores population with the population of the more evolved young stellar objects identified in the Spitzer surveys, we conclude that in Lupus 3 the bulk of the star formation activity has already passed and only a moderate number of stars are still forming. On the contrary, in Lupus 1 star formation is on-going and several dense cores are still in the pre--/proto--stellar phase. Lupus 4 is at an intermediate stage, with a smaller number of individual objects.

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arXiv2011

Filaments and ridges in Vela C revealed by Herschel: from low-mass to high-mass star-forming sites

We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 um, spanning the peak of emission of cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the `Centre-Ridge' and `South-Nest' sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments.

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arXiv2010

Source extraction and photometry for the far-infrared and sub-millimeter continuum in the presence of complex backgrounds

(Abridged) We present a new method for detecting and measuring compact sources in conditions of intense, and highly variable, fore/background. While all most commonly used packages carry out the source detection over the signal image, our proposed method builds from the measured image a "curvature" image by double-differentiation in four different directions. In this way point-like as well as resolved, yet relatively compact, objects are easily revealed while the slower varying fore/background is greatly diminished. Candidate sources are then identified by looking for pixels where the curvature exceeds, in absolute terms, a given threshold; the methodology easily allows us to pinpoint breakpoints in the source brightness profile and then derive reliable guesses for the sources extent. Identified peaks are fit with 2D elliptical Gaussians plus an underlying planar inclined plateau, with mild constraints on size and orientation. Mutually contaminating sources are fit with multiple Gaussians simultaneously using flexible constraints. We ran our method on simulated large-scale fields with 1000 sources of different peak flux overlaid on a realistic realization of diffuse background. We find detection rates in excess of 90% for sources with peak fluxes above the 3-sigma signal noise limit; for about 80% of the sources the recovered peak fluxes are within 30% of their input values.

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arXiv2009

Properties of Stellar Clusters around High-Mass Young Stars

[Abridged] Twenty-six high-luminosity IRAS sources believed to be collection of stars in the early phases of high-mass star formation have been observed in the NIR (J, H, K) to characterize the clustering properties of their young stellar population and gain insight into the initial conditions of star formation in these clusters (Initial Mass Function [IMF], Star Formation History [SFH]), and to deduce mean values for cluster ages. K luminosity functions (KLFs) are compared with simulated ones from a model that generates populations of synthetic clusters starting from assumptions on the IMF, the SFH, and the Pre-MS evolution, and using the average properties of the observed clusters as boundary conditions Twenty-two sources show evidence of clustering from a few up to several tens of objects, and a median cluster radius of 0.7 pc. A considerable number of cluster members present an infrared excess characteristic of young Pre-Main-Sequence objects. We find that the median stellar age ranges between 2.5 10^5 and 5 10^6 years, with evidence of an age spread of the same entity within each cluster. We also find evidence that older clusters tend to be smaller in size, in line with the fact that our clusters are on average larger than those around relatively older Herbig Ae/Be stars. The relationship of the mass of the most massive star in the cluster with both the clusters richness and their total stellar mass suggest that our modeled clusters may not be consistent with them resulting from random sampling of the IMF. Our results are consistent with a star formation which takes place continuously over a period of time which is longer than a typical crossing time.

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arXiv2005

Search for massive protostar candidates in the southern hemisphere: II. Dust continuum emission

In an ongoing effort to identify and study high-mass protostellar candidates we have observed in various tracers a sample of 235 sources selected from the IRAS Point Source Catalog, mostly with dec < -30 deg, with the SEST antenna at millimeter wavelengths. The sample contains 142 Low sources and 93 High, which are believed to be in different evolutionary stages. Both sub-samples have been studied in detail by comparing their physical properties and morphologies. Massive dust clumps have been detected in all but 8 regions, with usually more than one clump per region. The dust emission shows a variety of complex morphologies, sometimes with multiple clumps forming filaments or clusters. The mean clump has a linear size of ~0.5 pc, a mass of ~320 Msolar for a dust temperature Td=30 K, an H_2 density of 9.5E5 cm-3, and a surface density of 0.4 g cm-2. The median values are 0.4 pc, 102 Msolar, 4E4 cm-3, and 0.14 g cm-2, respectively. The mean value of the luminosity-to-mass ratio, L/M ~99 Lsolar/Msolar, suggests that the sources are in a young, pre-ultracompact HII phase. We have compared the millimeter continuum maps with images of the mid-IR MSX emission, and have discovered 95 massive millimeter clumps non-MSX emitters, either diffuse or point-like, that are potential prestellar or precluster cores. The physical properties of these clumps are similar to those of the others, apart from the mass that is ~3 times lower than for clumps with MSX counterpart. Such a difference could be due to the potential prestellar clumps having a lower dust temperature. The mass spectrum of the clumps with masses above M ~100 Msolar is best fitted with a power-law dN/dM proportional to M-alpha with alpha=2.1, consistent with the Salpeter (1955) stellar IMF, with alpha=2.35.

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arXiv2001

Far Infrared Spectroscopy of the HH 1/2 Outflow

The HH 1/2 system has been observed with the two spectrometers on board the Infrared Space Observatory. Diffuse [CII] 158um emission indicate the presence of a Photo-Dissociation Region (PDR) which is only in part due to the external Far-UV irradiation from the nearby Orion Nebula. Additional irradiation must be originated locally and we show that the FUV field produced in the recombination regions behind the shocks traced by the HH objects is sufficient to induce a PDR at the flow cavity walls. The analysis of [OI], [SiII]34.8um and [NeII] 12.8um lines suggests shock velocities v=100-140 km/s with pre-shock densities between 100 and 1000 per cc. H2 pure rotational lines trace a T=600 K gas which is likely to be warmed up in slow, planar, molecular shocks. The coexistence in the areas subtended by the instrumental beamsizes of shocks at different velocities is consistent with the bow-shock morphology of the HH 1 and, to a lesser extent, HH 2 as traced by optical images.

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arXiv2000

A Shock-Induced PDR in the HH 80/81 Flow. Far Infrared Spectroscopy

The two spectrometers on board the Infrared Space Observatory were used to observe the Herbig-Haro objects HH 80, 81 and 80N, as well as their candidate exciting source IRAS 18162-2048. The fine structure lines of [OI]63um, [OI]145um and [CII]158um are detected everywhere, while [NII]122um and [OIII]88um are only detected toward the HH objects; line ratios confirm for the first time the collisionally excited HH nature of HH 80N. No molecular line is detected in any of the observed positions. We use a full shock code to diagnose shock velocities vs~100 km/s toward the HH objects, as expected from the optical spectroscopy. Since proper motions suggest velocities in excess of 600 km/s, the HH objects probably represent the interface between two flow components with velocity differing by ~vs. Aside from the flow exciting source, the [CII]158um line is everywhere brighter than the [OI]63um line, indicating the presence of a Photo-Dissociation Region (PDR) all along the flow. Continuum emission from the HH objects and from other positions along the flow is only detected longword of ~50 micron, and its proportionality to the [CII]158um line flux suggests it is PDR in origin. We propose that the FUV continuum irradiated by the HH objects and the jet is responsible for the generation of a PDR at the walls of the flow cavity. We develop a very simple model which strengthens the plausibility of this hypothesis.

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arXiv2000

A search for precursors of Ultracompact HII Regions in a sample of luminous IRAS sources. III: Circumstellar Dust Properties

The James Clerk Maxwell Telescope has been used to obtain submillimeter and millimeter continuum photometry of a sample of 30 IRAS sources previously studied in molecular lines and centimeter radio continuum. All the sources have IRAS colours typical of very young stellar objects (YSOs) and are associated with dense gas. In spite of their high luminosities (L>10000 solar units), only ten of these sources are also associated with a radio counterpart. In 17 cases we could identify a clear peak of millimeter emission associated with the IRAS source, while in 9 sources the millimeter emission was extended or faint and a clear peak could not be identified; upper limits were found in 4 cases only. Using simple greybody fitting model to the observed SED, we derive global properties of the circumstellar dust. The dust temperature varies from 24 K to 45 K, while the exponent of the dust emissivity vs frequency power-law spans a range 1.56<beta<2.38, characteristic of silicate dust; total circumstellar masses range up to more than 500 solar masses. We find that for sources with comparable luminosities, the total column densities derived from the dust masses do not distinguish between sources with and without radio counterpart. We interpret this result as an indication that dust does not play a dominant role in inhibiting the formation of the HII region. We examine several scenarios for their origin in terms of newborn ZAMS stars and although most of these fail to explain the observations, we cannot exclude that these sources are young stars already on the ZAMS with modest residual accretion that quenches the expansion of the HII region. Finally, we consider the possibility that the IRAS sources are high-mass pre-ZAMS (or pre-H-burning) objects deriving most of the emitted luminosity from accretion.

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arXiv1999

Detection of the 62 micron Crystalline Water Ice Feature in Emission toward HH7 with ISO-LWS

We report the detection of the 62 micron feature of crystalline water ice in emission towards the bow-shaped Herbig-Haro object HH 7. Significant amounts of far infrared continuum emission are also detected between 10 and 200 micron, so that Herbig-Haro objects cease to be pure emission-line objects at FIR wavelengths. The formation of crystalline water ice mantles requires grain temperatures T > 100 K at the time of mantle formation, suggesting that we are seeing material processed by the HH 7 shock front. The deduced ice mass is \~2e-5 solar masses corresponding to a water column density N~ 1e18 cm-3; an estimate of the [H2O]/[H] abundance yields values close to the interstellar gas-phase oxygen abundance. The relatively high dust temperature and the copious amounts of gas-phase water needed to produce the observed quantity of crystalline water ice, suggest a scenario where both dissociative and non-dissociative shocks co-exist. The timescale for ice mantle formation is of the order of ~400 years, so that the importance of gas-phase water cooling as a shock diagnostic may be greatly diminished.

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arXiv1998

IRAS 23385+6053: A Prototype Massive Class 0 Object

IRAS 23385+6053 is a Young Stellar Object with luminosity 16,000 solar luminosities at a kinematic distance of 4.9 kpc. This candidate precursor of an ultracompact HII region is associated with a millimeter source detected at the JCMT but is undetected at centimeter wavelengths with the VLA. We observed this source with the OVRO millimeter array at 3.4 mm in the continuum, HCO+(1-0), H13CO+(1-0) and SiO(v=0, 2-1) line emission, and with CAM aboard ISO at 6.75 and 15 um. The IRAS source is coincident with a 3.4 mm compact (r~0.048 pc) and massive (M~370 solar masses) core, which is undetected at 15 um to a 3 sigma level of 6 mJy; this is compatible with the derived H2 column density of 2x10(24) and the estimated visual extinction of about 2000 mag. We find L(submm)/L(bol)>10(-3) and M(env)/M(star)>>1, typical of Class 0 objects. The source is also associated with a compact outflow characterized by a size similar to the core radius, a dynamical timescale of about 7,000 years, and a mass loss rate of about 10(-3) solar masses/year. The axis of the outflow is oriented nearly perpendicular to the plane of the sky, ruling out the possibility that the non-detection at 15 um is the result of a geometric effect. All these properties suggest that IRAS 23385+6053 is the first example of a bona fide massive Class 0 object.

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arXiv1998

ISOCAM Molecular Hydrogen Images of the Cep E Outflow

The physical characteristics of Cepheus E (Cep E) `embedded' outflow are analyzed using ISOCAM images in the v=0-0 S(5) 6.91 um and S(3) 9.66 um molecular hydrogen lines. We find that the morphology of the Cep E outflow in the ground vibrational H2 lines is similar to that of the near infrared v=1-0 2.12 um line. At these mid-IR wavelengths, we do not detect the second H2 outflow which is almost perpendicular to Cep E 2.121 um flow or traces of H2 emission along the second 12CO J = 2-1 outflow at 52 degrees angle, down to a surface brightness of 12 - 46 uJy/arcsec square. We do detect at 6.91 um the likely source of the main H2 and CO outflows, IRAS 23011+6126, and show that the source is easily seen in all IRAS bands using HiRes images. The source is not detected at 9.66 um, but we think this agrees with the interstellar extinction curve which has a minimum at 7 um, but rises a 9.7 um due to the strong absorption silicate feature, enhanced in this case by a cocoon surrounding the Class 0 object. This idea is supported by our models of the spectral energy distribution (SED) of the central object. The models assume that the main source of opacity is due to bare silicates and our best fit for the SED yields a total mass of envelope of 17 solar masses and a dust temperature of 18 K.

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