arXiv subjects

S. Molinari

Publications and source records attributed to S. Molinari.

At least 109 records · Page 6Linked to original sources

arXiv2014

A Herschel and BIMA study of the sequential star formation near the W48A HII region

We present the results of Herschel HOBYS photometric mapping combined with BIMA observations and additional archival data, and perform an in-depth study of the evolutionary phases of the star-forming clumps in W 48A and their surroundings. Age estimates for the compact sources were derived from bolometric luminosities and envelope masses, which were obtained from the dust continuum emission, and agree within an order of magnitude with age estimates from molecular line and radio data. The clumps in W 48A are linearly aligned by age (east-old to west-young): we find a ultra compact (UC) HII region, a young stellar object (YSO) with class II methanol maser emission, a YSO with a massive outflow, and finally the NH_2D prestellar cores from Pillai et al. This remarkable positioning reflects the (star) formation history of the region. We find that it is unlikely that the star formation in the W 48A molecular cloud was triggered by the UCHII region and discuss the Aquila supershell expansion as a mayor influence on the evolution of W 48A. We conclude that the combination of Herschel continuum data with interferometric molecular line and radio continuum data is important to derive trustworthy age estimates and interpret the origin of large scale structures through kinematic information.

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arXiv2014

A sub-arcsecond study of the hot molecular core in G023.01-00.41

(Abridged) METHODS: We performed SMA observations at 1.3 mm with both the most extended and compact array configurations, providing sub-arcsecond and high sensitivity maps of various molecular lines, including both hot-core and outflow tracers. We also reconstruct the spectral energy distribution of the region from millimeter to near infrared wavelengths, using the Herschel/Hi-GAL maps, as well as archival data. RESULTS: From the spectral energy distribution, we derive a bolometric luminosity of about 4x10^4 Lsun. Our interferometric observations reveal that the distribution of dense gas and dust in the HMC is significantly flattened and extends up to a radius of 8000 AU from the center of radio continuum and maser emission in the region. The equatorial plane of this HMC is strictly perpendicular to the elongation of the collimated bipolar outflow, as imaged on scales of about 0.1-0.5 pc in the main CO isotopomers as well as in the SiO(5-4) line. In the innermost HMC regions (ca. 1000 AU), the velocity field traced by the CH3CN(12_K-11_K) line emission shows that molecular gas is both expanding along the outflow direction following a Hubble-law, and rotating about the outflow axis, in agreement with the (3-D) velocity field traced by methanol masers. The velocity field associated with rotation indicates a dynamical mass of 19 Msun at the center of the core. The latter is likely to be concentrated in a single O9.5 ZAMS star, consistent with the estimated bolometric luminosity of G023.01-00.41. The physical properties of the CO(2-1) outflow emission, such as its momentum rate 6x10^-3 Msun km/s /yr and its outflow rate 2x10^-4 Msun/yr, support our estimates of the luminosity (and mass) of the embedded young stellar object.

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arXiv2013

On the shape of the mass-function of dense clumps in the Hi-GAL fields. II. Using Bayesian inference to study the clump mass function

Context. Stars form in dense, dusty clumps of molecular clouds, but little is known about their origin, their evolution and their detailed physical properties. In particular, the relationship between the mass distribution of these clumps (also known as the "clump mass function", or CMF) and the stellar initial mass function (IMF), is still poorly understood. Aims. In order to better understand how the CMF evolve toward the IMF, and to discern the "true" shape of the CMF, large samples of bona-fide pre- and proto-stellar clumps are required. Two such datasets obtained from the Herschel infrared GALactic Plane Survey (Hi-GAL) have been described in paper I. Robust statistical methods are needed in order to infer the parameters describing the models used to fit the CMF, and to compare the competing models themselves. Methods. In this paper we apply Bayesian inference to the analysis of the CMF of the two regions discussed in Paper I. First, we determine the Bayesian posterior probability distribution for each of the fitted parameters. Then, we carry out a quantitative comparison of the models used to fit the CMF. Results. We have compared the results from several methods implementing Bayesian inference, and we have also analyzed the impact of the choice of priors and the influence of various constraints on the statistical conclusions for the preferred values of the parameters. We find that both parameter estimation and model comparison depend on the choice of parameter priors. Conclusions. Our results confirm our earlier conclusion that the CMFs of the two Hi-GAL regions studied here have very similar shapes but different mass scales. Furthermore, the lognormal model appears to better describe the CMF measured in the two Hi-GAL regions studied here. However, this preliminary conclusion is dependent on the choice of parameters priors.

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arXiv2013

Discovery of weak 6.7-GHz CH3OH masers in a sample of high-mass Hi-GAL sources

Maser lines from different molecular species, including water, hydroxyl, and methanol, are common observational phenomena associated with massive star forming regions. In particular, the methanol maser appears as an ideal tool to study the early phases of massive star formation. However, it is difficult to establish the exact start of the methanol maser phase, and it would then be interesting to detect and study low-flux density methanol masers (i.e., < 0.1 Jy or even << 0.1 Jy), in order to determine if they can effectively be used to mark a specific evolutionary phase in high-mass star formation. Past surveys have been unable to systematically detect many low-flux density methanol masers, and thus we do not yet know how many such masers exist in the Galaxy and what is their physical nature. Out of a sample of 107 observed Hi-GAL sources we detected a total of 32 methanol masers, with 22 sources being new and weak (median peak flux density 0.07 Jy) detections, in the Galactic longitude range [32.0, 59.8]deg. We also detected 12 6.035-GHz OH maser, with 9 objects being new detections. Our survey covers a similar range of source distances as the "Arecibo Methanol Maser Galactic Plane Survey", but the methanol masers detected by us are clearly shifted towards lower integrated flux densities. The newly detected methanol masers are mostly of low-luminosity and, except for some sources, their weakness is not due to distance effects or positional offsets. No specific correlation is found with the physical parameters of the Hi-GAL clumps, except for sources with both CH3OH and OH masers which tend to have higher mass and luminosity. The intensity of the methanol masers correlates well with the velocity range of the maser emission, which suggests that the low brightness of these masers is related to the number of maser spots in the emitting region and their evolution with time.

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arXiv2013

Two Mass Distributions in the L 1641 Molecular Clouds: The Herschel connection of Dense Cores and Filaments in Orion A

We present the Herschel Gould Belt survey maps of the L1641 molecular clouds in Orion A. We extracted both the filaments and dense cores in the region. We identified which of dense sources are proto- or pre-stellar, and studied their association with the identified filaments. We find that although most (71%) of the pre-stellar sources are located on filaments there is still a significant fraction of sources not associated with such structures. We find that these two populations (on and off the identified filaments) have distinctly different mass distributions. The mass distribution of the sources on the filaments is found to peak at 4 Solar masses and drives the shape of the CMF at higher masses, which we fit with a power law of the form dN/dlogM \propto M^{-1.4+/-0.4}. The mass distribution of the sources off the filaments, on the other hand, peaks at 0.8 Solar masses and leads to a flattening of the CMF at masses lower than ~4 Solar masses. We postulate that this difference between the mass distributions is due to the higher proportion of gas that is available in the filaments, rather than in the diffuse cloud.

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arXiv2013

Physical properties of high-mass clumps in different stages of evolution

(Abridged) Aims. To investigate the first stages of the process of high-mass star formation, we selected a sample of massive clumps previously observed with the SEST at 1.2 mm and with the ATNF ATCA at 1.3 cm. We want to characterize the physical conditions in such sources, and test whether their properties depend on the evolutionary stage of the clump. Methods. With ATCA we observed the selected sources in the NH3(1,1) and (2,2) transitions and in the 22 GHz H2O maser line. Ammonia lines are a good temperature probe that allow us to accurately determine the mass and the column-, volume-, and surface densities of the clumps. We also collected all data available to construct the spectral energy distribution of the individual clumps and to determine if star formation is already occurring, through observations of its most common signposts, thus putting constraints on the evolutionary stage of the source. We fitted the spectral energy distribution between 1.2 mm and 70 microns with a modified black body to derive the dust temperature and independently determine the mass. Results. The clumps are cold (T~10-30 K), massive (M~10^2-10^3 Mo), and dense (n(H2)>~10^5 cm^-3) and they have high column densities (N(H2)~10^23 cm^-2). All clumps appear to be potentially able to form high-mass stars. The most massive clumps appear to be gravitationally unstable, if the only sources of support against collapse are turbulence and thermal pressure, which possibly indicates that the magnetic field is important in stabilizing them. Conclusions. After investigating how the average properties depend on the evolutionary phase of the source, we find that the temperature and central density progressively increase with time. Sources likely hosting a ZAMS star show a steeper radial dependence of the volume density and tend to be more compact than starless clumps.

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arXiv2013

Global collapse of molecular clouds as a formation mechanism for the most massive stars

The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(+/-800) Msun infrared dark cloud SDC335.579-0.272 (hereafter SDC335) which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(+770,-385) Msun contained within a source diameter of 0.05pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity V_{inf} =0.7(+/-0.2) km/s, and a total mass infall rate \dot{M}_{inf} = 2.5(+/-1.0) x 10^{-3} Msun/yr towards the central pc-size region of SDC335. This infall rate brings 750(+/-300) Msun of gas to the centre of the cloud per free-fall time (t_{ff}=3x10^5 yr). This is enough to double the mass already present in the central pc-size region in 3.5(+2.2,-1.0) x t_{ff}. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud's gravitational potential well.

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arXiv2013

Herschel observations of the Sgr B2 cores: Hydrides, warm CO, and cold dust

Sagittarius B2 (Sgr B2) is one of the most massive and luminous star-forming regions in the Galaxy and shows chemical and physical conditions similar to those in distant extragalactic starbursts. We present large-scale far-IR/submm photometric images and spectroscopic maps taken with the PACS and SPIRE instruments onboard Herschel. The spectra towards the Sgr B2 star-forming cores, B2(M) and B2(N), are characterized by strong CO line emission, emission lines from high-density tracers (HCN, HCO+, and H2S), [N II] 205 um emission from ionized gas, and absorption lines from hydride molecules (OH+, H2O+, H2O, CH+, CH, SH+, HF, NH, NH2, and NH3). The rotational population diagrams of CO suggest the presence of two gas temperature components: an extended warm component, which is associated with the extended envelope, and a hotter component, which is seen towards the B2(M) and B2(N) cores. As observed in other Galactic Center clouds, the gas temperatures are significantly higher than the dust temperatures inferred from photometric images. We determined far-IR and total dust masses in the cores. Non-local thermodynamic equilibrium models of the CO excitation were used to constrain the averaged gas density in the cores. A uniform luminosity ratio is measured along the extended envelope, suggesting that the same mechanism dominates the heating of the molecular gas at large scales. The detection of high-density molecular tracers and of strong [N II] 205 um line emission towards the cores suggests that their morphology must be clumpy to allow UV radiation to escape from the inner HII regions. Together with shocks, the strong UV radiation field is likely responsible for the heating of the hot CO component. At larger scales, photodissociation regions models can explain both the observed CO line ratios and the uniform L(CO)/LFIR luminosity ratios.

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arXiv2013

Herschel Reveals Massive Cold Clumps in NGC 7538

We present the first overview of the Herschel observations of the nearby high-mass star-forming region NGC 7538, taken as part of the Herschel imaging study of OB Young Stellar objects (HOBYS) Key Programme. These PACS and SPIRE maps cover an approximate area of one square degree at five submillimeter and far-infrared wavebands. We have identified 780 dense sources and classified 224 of those. With the intention of investigating the existence of cold massive starless or class 0-like clumps that would have the potential to form intermediate- to high-mass stars, we further isolate 13 clumps as the most likely candidates for followup studies. These 13 clumps have masses in excess of 40 M_sun and temperatures below 15 K. They range in size from 0.4 pc to 2.5 pc and have densities between 3x10^3 cm^-3 to 4x10^4 cm^-3. Spectral energy distributions are then used to characterize their energetics and evolutionary state through a luminosity-mass diagram. NGC 7538 has a highly filamentary structure, previously unseen in the dust continuum of existing submillimeter surveys. We report the most complete imaging to date of a large, evacuated ring of material in NGC 7538 which is bordered by many cool sources.

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arXiv2013

Evolution and excitation conditions of outflows in high-mass star-forming regions

Theoretical models suggest that massive stars form via disk-mediated accretion, with bipolar outflows playing a fundamental role. A recent study toward massive molecular outflows has revealed a decrease of the SiO line intensity as the object evolves. The present study aims at characterizing the variation of the molecular outflow properties with time, and at studying the SiO excitation conditions in outflows associated with massive YSOs. We used the IRAM30m telescope to map 14 massive star-forming regions in the SiO(2-1), SiO(5-4) and HCO+(1-0) outflow lines, and in several dense gas and hot core tracers. Hi-GAL data was used to improve the spectral energy distributions and the L/M ratio, which is believed to be a good indicator of the evolutionary stage of the YSO. We detect SiO and HCO+ outflow emission in all the sources, and bipolar structures in six of them. The outflow parameters are similar to those found toward other massive YSOs. We find an increase of the HCO+ outflow energetics as the object evolve, and a decrease of the SiO abundance with time, from 10^(-8) to 10^(-9). The SiO(5-4) to (2-1) line ratio is found to be low at the ambient gas velocity, and increases as we move to high velocities, indicating that the excitation conditions of the SiO change with the velocity of the gas (with larger densities and/or temperatures for the high-velocity gas component). The properties of the SiO and HCO+ outflow emission suggest a scenario in which SiO is largely enhanced in the first evolutionary stages, probably due to strong shocks produced by the protostellar jet. As the object evolves, the power of the jet would decrease and so does the SiO abundance. During this process, however, the material surrounding the protostar would have been been swept up by the jet, and the outflow activity, traced by entrained molecular material (HCO+), would increase with time.

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arXiv2013

The first Hi-GAL observations of the outer Galaxy: a look to star formation in the third Galactic quadrant in the longitude range 216.5 < l < 225.5

We present the first Herschel PACS and SPIRE photometric observations in a portion of the outer Galaxy ($216.5^{\circ} \lesssim \ell \lesssim 225.5^{\circ}$ and $-2^{\circ} \lesssim b \lesssim 0^{\circ}$) as a part of the Hi-GAL survey. The maps between 70 and 500 $μ$m, the derived column density and temperature maps, and the compact source catalog are presented. NANTEN CO(1-0) line observations are used to derive cloud kinematics and distances, so that we can estimate distance-dependent physical parameters of the compact sources (cores and clumps) having a reliable spectral energy distribution, that we separate in 255 proto-stellar and 688 starless. Both typologies are found in association with all the distance components observed in the field, up to $\sim 5.8$ kpc, testifying the presence of star formation beyond the Perseus arm at these longitudes. Selecting the starless gravitationally bound sources we identify 590 pre-stellar candidates. Several sources of both proto- and pre-stellar nature are found to exceed the minimum requirement for being compatible with massive star formation, based on the mass-radius relation. For the pre-stellar sources belonging to the Local arm ($d\lesssim1.5$ kpc) we study the mass function, whose high-mass end shows a power-law $N(\log M) \propto M^{-1.0 \pm 0.2}$. Finally, we use a luminosity vs mass diagram to infer the evolutionary status of the sources, finding that most of the proto-stellar are in the early accretion phase (with some cases compatible with a Class I stage), while for pre-stellar sources, in general, accretion has not started yet.

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arXiv2013

Candidate super star cluster progenitor gas clouds possibly triggered by close passage to Sgr A*

Super star clusters are the end product of star formation under the most extreme conditions. As such, studying how their final stellar populations are assembled from their natal progenitor gas clouds can provide strong constraints on star formation theories. An obvious place to look for the initial conditions of such extreme stellar clusters are gas clouds of comparable mass and density, with no star formation activity. We present a method to identify such progenitor gas clouds and demonstrate the technique for the gas in the inner few hundred pc of our Galaxy. The method highlights three clouds in the region with similar global physical properties to the previously identified extreme cloud, G0.253+0.016, as potential young massive cluster (YMC) precursors. The fact that four potential YMC progenitor clouds have been identified in the inner 100 pc of the Galaxy, but no clouds with similar properties have been found in the whole first quadrant despite extensive observational efforts, has implications for cluster formation/destruction rates across the Galaxy. We put forward a scenario to explain how such dense gas clouds can arise in the Galactic centre environment, in which YMC formation is triggered by gas streams passing close to the minimum of the global Galactic gravitational potential at the location of the central supermassive black hole, Sgr A*. If this triggering mechanism can be verified, we can use the known time interval since closest approach to Sgr A* to study the physics of stellar mass assembly in an extreme environment as a function of absolute time.

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arXiv2013

A Hi-GAL study of the high-mass star-forming region G29.96-0.02

Context. G29.96-0.02 is a high-mass star-forming cloud observed at 70, 160, 250, 350, and 500 microns as part of the Herschel survey of the Galactic Plane during the Science Demonstration Phase. Aims. We wish to conduct a far-infrared study of the sources associated with this star-forming region by estimating their physical properties and evolutionary stage, and investigating the clump mass function, the star formation efficiency and rate in the cloud. Methods. We have identified the Hi-GAL sources associated with the cloud, searched for possible counterparts at centimeter and infrared wavelengths, fitted their spectral energy distribution and estimated their physical parameters. Results. A total of 198 sources have been detected in all 5 Hi-GAL bands, 117 of which are associated with 24 microns emission and 87 of which are not associated with 24 microns emission. We called the former sources 24 microns-bright and the latter ones 24 microns-dark. The [70-160] color of the 24 microns-dark sources is smaller than that of the 24 microns-bright ones. The 24 microns-dark sources have lower L_bol and L_bol/M_env than the 24 microns-bright ones for similar M_env, which suggests that they are in an earlier evolutionary phase. The G29-SFR cloud is associated with 10 NVSS sources and with extended centimeter continuum emission well correlated with the 70 microns emission. Most of the NVSS sources appear to be early B or late O-type stars. The most massive and luminous Hi-GAL sources in the cloud are located close to the G29-UC region, which suggests that there is a privileged area for massive star formation towards the center of the G29-SFR cloud. Almost all the Hi-GAL sources have masses well above the Jeans mass but only 5% have masses above the virial mass, which indicates that most of the sources are stable against gravitational collapse. The sources with M_env > M_virial and that ...

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arXiv2012

Herschel view of the Taurus B211/3 filament and striations: Evidence of filamentary growth?

We present first results from the Herschel Gould Belt survey for the B211/L1495 region in the Taurus molecular cloud. Thanks to their high sensitivity and dynamic range, the Herschel images reveal the structure of the dense, star-forming filament B211 with unprecedented detail, along with the presence of striations perpendicular to the filament and generally oriented along the magnetic field direction as traced by optical polarization vectors. Based on the column density and dust temperature maps derived from the Herschel data, we find that the radial density profile of the B211 filament approaches a power-law behavior ρ {\propto} r^(-2.0{\pm}0.4) at large radii and that the temperature profile exhibits a marked drop at small radii. The observed density and temperature profiles of the B211 filament are in good agreement with a theoretical model of a cylindrical filament undergoing gravitational contraction with a polytropic equation of state: P {\propto} ρ^γ and T {\propto} ρ^(γ-1), with γ=0.97{\pm}0.01<1 (i.e. not strictly isothermal). The morphology of the column density map, where some of the perpendicular striations are apparently connected to the B211 filament, further suggests that the material may be accreting along the striations onto the main filament. The typical velocities expected for the infalling material in this picture are ~0.5-1 km/s, which are consistent with the existing kinematical constraints from previous CO observations.

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arXiv2012

Recent star formation in the Lupus clouds as seen by Herschel

We present a study of the star formation histories of the Lupus I, III, and IV clouds using the Herschel 70-500 micron maps obtained by the Herschel Gould Belt Survey Key Project. By combining the new Herschel data with the existing Spitzer catalog we obtained an unprecedented census of prestellar sources and young stellar objects in the Lupus clouds, which allowed us to study the overall star formation rate (SFR) and efficiency (SFE). The high SFE of Lupus III, its decreasing SFR, and its large number of pre-main sequence stars with respect to proto- and prestellar sources, suggest that Lupus III is the most evolved cloud, and after having experienced a major star formation event in the past, is now approaching the end of its current star-forming cycle. Lupus I is currently undergoing a large star formation event, apparent by the increasing SFR, the large number of prestellar objects with respect to more evolved objects, and the high percentage of material at high extinction (e.g., above A_V=8 mag). Also Lupus IV has an increasing SFR; however, the relative number of prestellar sources is much lower, suggesting that its star formation has not yet reached its peak.

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arXiv2012

An analysis of star formation with Herschel in the Hi-GAL Survey. I. The Science Demonstration Phase Fields

The Herschel survey of the Galactic Plane (Hi-GAL) provides a unique opportunity to study star formation over large areas of the sky and different environments in the Milky Way. We use the best studied Hi-GAL fields to date, two 2x2 tiles centered on (l, b) = (30, 0) deg and (l, b) = (59, 0) deg, to study the star formation activity using a large sample of well selected young stellar objects (YSOs). We estimate the star formation rate (SFR) for these fields using the number of candidate YSOs and their average time scale to reach the Zero Age Main Sequence, and compare it with the rate estimated using their integrated luminosity at 70 micron combined with an extragalactic star formation indicator. We measure a SFR of (9.5 +- 4.3)*10^{-4} Msol/yr and (1.6 +- 0.7)*10^{-4} Msol/yr with the source counting method, in l=30 deg and l=59 deg, respectively. Results with the 70 micron estimator are (2.4 +- 0.4)*10^{-4} Msol/yr and (2.6 +- 1.1)*10^{-6} Msol/yr. Since the 70 micron indicator is derived from averaging extragalactic star forming complexes, we perform an extrapolation of these values to the whole Milky Way and obtain SFR_{MW} = (0.71 +- 0.13) Msol/yr from l = 30 deg and SFR_{MW} = (0.10 +- 0.04) Msol/yr from l=59 deg. The estimates in l=30 deg are in agreement with the most recent results on the Galactic star formation activity, indicating that the characteristics of this field are likely close to those of the star-formation dominated galaxies used for its derivation. Since the sky coverage is limited, this analysis will improve when the full Hi-GAL survey will be available.

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arXiv2012

Different evolutionary stages in massive star formation. Centimeter continuum and H2O maser emission with ATCA

We present ATCA observations of the H2O maser line and radio continuum at 18.0GHz and 22.8GHz, toward a sample of 192 massive star forming regions containing several clumps already imaged at 1.2mm. The main aim of this study is to investigate the water maser and centimeter continuum emission (likely tracing thermal free-free emission) in sources at different evolutionary stages, using the evolutionary classifications proposed by Palla et al (1991) and Molinari et al (2008). We used the recently comissioned CABB backend at ATCA obtaining images with 20arcsec resolution in the 1.3cm continuum and H2O maser emission, in all targets. For the evolutionary analysis of the sources we used the millimeter continuum emission from Beltran et al (2006) and the infrared emission from the MSX Point Source Catalogue. We detect centimeter continuum emission in 88% of the observed fields with a typical rms noise level of 0.45mJy/beam. Most of the fields show a single radio continuum source, while in 20% of them we identify multiple components. A total of 214 centimeter continuum sources have been identified, likely tracing optically thin HII regions, with physical parameters typical of both extended and compact HII regions. Water maser emission was detected in 41% of the regions, resulting in a total of 85 distinct components. The low angular (20arcsec) and spectral (14km/s) resolutions do not allow a proper analysis of the water maser emission, but suffice to investigate its association with the continuum sources. We have also studied the detection rate of HII regions in the two types of IRAS sources defined by Palla et (1991) on the basis of the IRAS colours: High and Low. No significant differences are found, with large detection rates (>90%) for both High and Low sources. We classify the millimeter and infrared sources in our fields in three evolutionary stages following the scheme presented by ...

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arXiv2012

Herschel observations of B1-bS and B1-bN: two first hydrostatic core candidates in the Perseus star-forming cloud

We report far-IR Herschel observations obtained between 70 $μ$m and 500 $μ$m of two star-forming dusty condensations, B1-bS and B1-bN, in the B1 region of the Perseus star-forming cloud. In the Western part of the Perseus cloud, B1-bS is the only source detected in all of the 6 PACS and SPIRE photometric bands without being visible in the Spitzer map at 24 $μ$m. B1-bN is clearly detected between 100 $μ$m and 250 $μ$m. We have fitted the spectral energy distributions of these sources to derive their physical properties, and find that a simple greybody model fails to reproduce the observed SEDs. At least a two-component model, consisting of a central source surrounded by a dusty envelope is required. The properties derived from the fit, however, suggest that the central source is not a Class 0 object. We then conclude that while B1-bS and B1-bN appear to be more evolved than a pre-stellar core, the best-fit models suggest that their central objects are younger than a Class 0 source. Hence, they may be good candidates to be examples of the first hydrostatic core phase. The projected distance between B1-bS and B1-bN is a few Jeans lengths. If their physical separation is close to this value, this pair would allow the mutual interactions between two forming stars at a very early stage of their evolution to be studied.

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