arXiv subjects

B. Nisini

Publications and source records attributed to B. Nisini.

138 records · Page 8Linked to original sources

arXiv2007

Star formation in the Vela Molecular Ridge. Large scale mapping of cloud D in the mm continuum

The Vela Molecular Ridge is one of the nearest intermediate-mass star forming regions, located within the galactic plane and outside the solar circle. Cloud D, in particular, hosts a number of small embedded young clusters. We present the results of a large-scale map in the dust continuum at 1.2 mm of a ~ 1deg x 1deg area within cloud D. The main aim of the observations was to obtain a complete census of cluster-forming cores and isolated (both high- and low-mass) young stellar objects in early evolutionary phases. The bolometer array SIMBA at SEST was used to map the dust emission in the region with a typical sensitivity of ~ 20 mJy/beam. This allows a mass sensitivity of ~ 0.2 Msun. The resolution is 24 arcsec, corresponding to ~ 0.08 pc, roughly the radius of a typical young embedded cluster in the region. The continuum map is also compared to a large scale map of CO(1-0) integrated emission. Using the CLUMPFIND algorithm, a robust sample of 29 cores has been obtained, spanning the size range 0.03 - 0.25 pc and the mass range 0.4 - 88 Msun. The most massive cores are associated both with red IRAS sources and with embedded young clusters, and coincide with CO(1-0) integrated emission peaks. The cores are distributed according to a mass spectrum ~ M^{-alpha} and a mass-versus-size relation ~ D^{x}, with alpha ~ 1.45 - 1.9 and x ~ 1.1 - 1.7. They appear to originate in the fragmentation of gas filaments seen in CO(1-0) emission and their formation is probably induced by expanding shells of gas. The core mass spectrum is flatter than the Initial Mass Function of the associated clusters in the same mass range, suggesting further fragmentation within the most massive cores. A threshold A_V ~ 12 mag seems to be required for the onset of star formation in the gas.

astro-ph↗

arXiv2006

Mapping molecular emission in Vela Molecular Ridge Cloud D

We present the 12CO(1-0) and 13CO(2-1) line maps obtained observing with the SEST a $\sim 1\degr \times 1\degr$ region of the Vela Molecular Ridge, Cloud D. This cloud is part of an intermediate-mass star forming region that is relatively close to the Sun. Our observations reveal, over a wide range of spatial scales (from $\sim 0.1$ to a few parsecs), a variety of dense structures such as arcs, filaments and clumps, that are in many cases associated with far-IR point-like sources, recognized as young stellar objects and embedded star clusters. The velocity field analysis highlights the presence of possible expanding shells, extending over several parsecs, probably related to the star forming activity of the cloud. Furthermore, the analysis of the line shapes in the vicinity of the far-IR sources allowed the detection of 13 molecular outflows. Considering a hierarchical scenario for the gas structure, a cloud decomposition was obtained for both spectral lines by means of the CLUMPFIND algorithm. The CLUMPFIND output has been discussed critically and a method is proposed to reasonably correct the list of the identified clumps. We find that the corresponding mass spectrum shows a spectral index $α\sim1.3 ÷2.0$ and the derived clump masses are below the corresponding virial masses. The mass-radius and velocity dispersion-radius relationships are also briefly discussed for the recovered clump population.

astro-ph↗

arXiv2006

Warm SiO gas in molecular bullets associated with protostellar outflows

In this paper we present the first SiO multiline analysis (from J=2-1 to J=11-10) of the molecular bullets along the outflows of the Class 0 sources L1448-mm and L1157-mm, obtained through observations with IRAM and JCMT. We have computed the main physical parameters in each bullet and compared them with other tracers of warm and dense gas and with models for the SiO excitation in shocks. We find that the bullets close to L1448--mm, associated with high velocity gas, have higher excitation conditions (n(H2) ~ 10^{6} cm^{-3}, T > 500 K) with respect to the L1157 bullets (n(H2) ~1-5 10^{5} cm^{-3}, T ~ 100-300 K). In both the sources, there is a clear evidence of the presence of velocity components having different excitation conditions, with the denser and/or warmer gas associated with the gas at the higher speed. In L1448 the bulk of the emission is due to the high-excitation and high velocity gas, while in L1157 most of the emission comes from the low excitation gas at ambient velocity. The observed velocity-averaged line ratios are well reproduced by shocks with speeds v_s larger than ~ 30 km/s and densities ~ 10^{5} - 10^{6} cm^{-3}. Plane-parallel shock models, however, fail to predict all the observed line profiles and in particular the very similar profiles shown by both low and high excitation lines. The overall observations support the idea that the L1157 clumps are shock interaction events older than the L1448 bullets close to the driving source. In the latter objects, the velocity structure and the variations of physical parameters with the velocity resemble very closely those found in optical/IR jets near the protostar, suggesting that similar launching and excitation mechanisms are also at the origin of collimated jets seen at millimetre wavelengths.

astro-ph↗

arXiv2006

Molecular line emission in HH54: a coherent view from near to far infrared

Aims. We present a detailed study of the infrared line emission (1-200 micron) in the Herbig-Haro object HH54. Our database comprises: high- (R~9000) and low- (R~600) resolution spectroscopic data in the near-infrared band (1-2.5 micron); mid-infrared spectrophotometric images (5-12 micron); and, far-IR (45-200 micron, R~200) spectra acquired with the ISO satellite. As a result, we provide the detection of and the absolute fluxes for more than 60 molecular features (mainly from H2 in the near- and mid-infrared and from H2O, CO and OH in the far-infrared) and 23 ionic lines. Methods. The H2 lines, coming from levels from v=0 to v=4 have been interpreted in the context of a state-of-art shock code, whose output parameters are adopted as input to a Large Velocity Gradient computation in order to interpret the FIR emission of CO, H2O and OH. Results. The H2 emission can be interpreted as originating in either steady-state J-type shocks or in quasi-steady J-type shocks with magnetic precursor. However, our multi-species analysis shows that only a model of a J-type shock with magnetic precursor (v_shock=18 km/s, n_H=10^4 cm^-3, B=100 microG, age=400 yr) can account for both the observed H2 emission and the CO and H2O lines. Such a model predicts a H2O abundance of ~ 7 10^-5, in agreement with estimations from other shock models for outflows associated with low mass protostars. We can exclude the possibility that the observed atomic lines arise in the same shock as the molecular lines, and give arguments in favour of the presence of a further high-velocity, fully dissociative shock component in the region. Finally, in view of the forthcoming spectroscopic facilities on board of the Herschel satellite, we provide predictions for H2O lines considered to be the most suitable for diagnostic purposes.

astro-ph↗

arXiv2006

Evidence for T Tauri-like emission in the EXor V1118 Ori from near-IR and X-ray data

We present a near-IR study of the EXor variable V1118 Ori, performed by following a slightly declining phase after a recent outburst. In particular, the near-IR (0.8 - 2.3 micron) spectrum, obtained for the first time, shows a large variety of emission features of the HI and HeI recombination and CO overtone. By comparing the observed spectrum with a wind model, a mass loss rate value is derived along with other parameters whose values are typical of an accreting T Tauri star. In addition, we have used X-ray data from the XMM archive, taken in two different epochs during the declining phase monitored in IR. X-ray emission (in the range 0.5 - 10 keV) permits to derive several parameters which confirm the T Tauri nature of the source. In the near-IR the object maintains a low visual extinction during all the activity phases, confirming that variable extinction does not contribute to brightness variations. The lack of both a significant amount of circumstellar material and any evidence of IR cooling from collimated jet/outflow driven by the source, indicates that, at least this member of the EXor class, is in a late stage of the Pre-Main Sequence evolution. In the X-ray regime, an evident fading is present, detected in the post-outburst phase, that cannot be reconciled with the presence of any absorbing material. This circumstance, combined with the persistence (in the pre- and post-outburst phases) of a temperature component at about 10 MK, suggests that accretion has some influence in regulating the coronal activity.

astro-ph↗

arXiv2005

H2 active jets in the near IR as a probe of protostellar evolution

We present a NIR analysis of a sample of H2 outflows from young embedded sources to compare the physical properties and cooling mechanisms of the different flows. The sample comprises 23 outflows driven by Class 0 and I sources having low-intermediate luminosity. We have obtained narrow band images in H2 2.12um and FeII 1.64um and spectroscopic observations in the range 1-2.5um. H2 line ratios have been used to estimate the visual extinction and average temperature of the molecular gas. Av values range from 2 to 15mag; average temperatures range between 2000 and 4000K. In several knots a stratification of temperatures is found with maximum values up to 5000K. Such a stratification is more commonly observed in those knots which also show FeII emission, while a thermalized gas at a single temperature is generally found in knots emitting only in molecular lines. Combining narrow band imaging with the parameters derived from the spectral analysis, we are able to measure the total luminosity of the H2 and FeII shocked regions in each flow. H2 is the major IR coolant with an average L(H2)/L(FeII) ratio of 10^{-2}. 83% of the sources have a L(H2)/L(bol) ratio of 0.04, irrespective of the Class of the driving source, while a smaller group of sources have L(H2)/L(bol) an order of magnitude smaller.Such a separation reveals the non-homogeneous behaviour of ClassI, where sources with very different outflow activity can be found. This is consistent with other studies showing that among ClassI one can find objects with different accretion properties and it demonstrates that the H2 power in the jet can be a powerful tool to identify the most active sources of this class.

astro-ph↗

arXiv2005

AO Assisted Near-IR Spectroscopy of SVS 13 and its Jet

We present long-slit H- and K-band spectroscopy of the low-mass outflow source SVS 13, obtained with the adaptive-optics assisted imager-spectrometer NACO on the VLT. With a spatial resolution of < 0.25 arcsec and a pixel scale of 0.027 arcsec we precisely establish the relative offsets of H2, [FeII], CO, HI and NaI components from the source continuum. The H2 and [FeII] peaks are clearly associated with the jet, while the CO, HI and NaI peaks are spatially unresolved and coincident with the source, as is expected for emission associated with accretion processes. The H2 profile along the slit is resolved into multiple components, which increase in size though decrease in intensity with distance from the source. This trend might be consistent with thermal expansion of packets of gas ejected during periods of increased accretion activity. Indeed, for the brightest component nearest the source, proper motion measurements indicate a tangential velocity of 0.028 arcsec/year. It therefore seems unlikely that this emission peak is associated with a stationary zone of warm gas at the base of the jet. However, the same can not be said for the [FeII] peak, for which we see no evidence for motion downwind, even though radial velocity measurements indicate that the emission is associated with higher jet velocities. We postulate that the [FeII] could be associated with a collimation shock at the base of the jet.

astro-ph↗

arXiv2004

Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy

Near IR spectra obtained with ISAAC at VLT, have been used to pose constraints on the evolutionary state and accretion properties of a sample of five embedded YSOs located in the R CrA core. This sample includes three Class I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6 and IRS3). Absorption lines have been detected in the medium resolution spectra of all the observed targets, together with emission lines likely originating in the disk-star-wind connected regions. We derived spectral types, veiling and stellar luminosity of the five observed sources, which in turn have been used to infer their mass and age adopting pre-main sequence evolutionary tracks. We find that in HH100 IR and IRS2 most of the bolometric luminosity is due to accretion, while the other three investigated sources, including the Class I object IRS5a, present a low accretion activity (L_{acc}/L_{bol} < 0.2). We observe a general correlation between the accretion luminosity, the IR veiling and the emission line activity of the sources. A correlation between the accretion activity and the spectral energy distribution slope is recognizable but with the notable exception of IRS5a. Our analysis therefore shows how the definition of the evolutionary stage of deeply embedded YSOs by means of IR colors needs to be more carefully refined.

astro-ph↗

arXiv2004

On the excitation of the infrared knots along protostellar jets

The complete near infrared (0.9-2.5 micron) spectra in three different star forming regions (HH24-26, HH72 and BHR71) are presented and analyzed in the framework of shock excitation models. The spectra are dominated by H_2 rovibrational emission (vibrational state nu up to 5, excitation energy T_ex up to 35000 K), while emission from ionized material, recognizable from [Fe II] and [S II] lines, is significantly fainter. The analysis of the H_2 excitation diagrams points to the existence of two different excitation regimes: whilst condensations observed only in the infrared appear to have temperatures rarely exceeding 3000 K and can be modelled in the framework of steady-state C-shock models, the infrared counterparts of Herbig Haro (HH) objects exhibit a temperature stratification with components up to more than 5000 K. The H_2 emission from representative HH objects (HH26A, HH72A and HH320A) has been successfully modelled by planar J-shocks with magnetic precursors, for which the main parameters (pre-shock density, speed) are derived. However, these same models are unable to reproduce the observed atomic and ionic emission, which probably arises from a distinct and perhaps more embedded region with respect to that traced by the H_2. Some of the physical parameters of such regions (fractional ionization, density) have been estimated in HH72, on the basis of the observed ionic lines.

astro-ph↗

arXiv2000

The ISO-LWS map of the Serpens cloud core: I. The SEDs of the IR/SMM sources

ISO-LWS mapping observations of the Serpens molecular cloud core are presented. The spectral range is 50 - 200 micron and the map size is 8' X 8'. These observations suffer from severe source confusion at FIR wavelengths and we employ a Maximum Likelihood Method for the spectro-spatial deconvolution. The strong and fairly isolated source SMM1 FIRS1 presented a test case, whose modelled spectral energy distribution (SED), within observational errors, is identical to the observed one. The model results for the other infrared and submillimetre sources are therefore likely to represent their correct SEDs. Simulations demonstrating the reliability and potential of the developed method support this view. For the majority of sources the peak of the SEDs is found within the spectral range of the LWS and derived temperatures are generally higher (>30K) than have been found by earlier deconvolution attempts using IRAS data. SMM sizes are found to be only a few arcsec in diameter. In addition, the SMMs are generally optically thick even at LWS wavelengths. Self-consistent radiative transfer calculations indicates that the SMMs are optically thick out to even longer wavelengths. Models were calculated for five sources, for which sufficient data were available, viz. SMM 1, 2, 3, 4 and 9. These models are optically thick out to millimetre wavelengths (wavelength of unit optical depth 900 to 1400 micron). Envelope masses for these SMMs are in the range 2-6 Msun, which is of course considerably more massive than estimates based on the optically thin assumption. The luminosities are in the range 10-70 Lsun, suggesting the formation of low-mass to intermediate mass stars, so that the existence of such massive envelopes argues for extreme youth of the SMMs in the Serpens cloud core.

astro-ph↗

arXiv2000

ISO Spectroscopy of the HH 7-11 Flow and its red-shifted Counterpart

We have used the two spectrometers on the Infrared Space Observatory to observe the HH 7-11 flow, its red-shifted counterpart, and the candidate exciting source SVS 13, in the star formation region NGC 1333. We detect atomic ([OI]63um and 145um, [SiII]34.8um, [CII]158um) and molecular (H_2, CO, H_2O) lines at various positions along the bipolar flow. Most of the observed lines can be explained in terms of shock-excited emission. In particular, our analysis shows that dissociative (J-type) and non-dissociative (C-type) shocks are simultaneously present everywhere along both lobes of the flow. We confirm the low-excitation nature of the Herbig-Haro nebulosities, with shock velocities <40-50 km/s. Toward both lobes of the outflow we find pre-shock densities of n_0~10^4 cm-3 for both the J and C components, implying B_0~100uG. In the central region of the flow, close to the exciting source, the pre-shock density deduced for the C-shock component is n_0~10^5 cm-3, suggesting a magnetic field ~3 times stronger. We propose that the deficiency of gas-phase water in the post C-shock regions is due to freezing onto warm grains processed through the J-shock front and traveling along the magnetic field lines. The total observed cooling from the dissociative shock components is consistent with the power lost by a slow molecular outflow accelerated by a fast neutral HI wind. Finally, the skin of the cloud seen in projection toward the flow appears to be weakly photo-ionised by BD +30 549, the dominant illuminating source of the NGC 1333 reflection nebula.

astro-ph↗

arXiv1999

Shock excited far-infrared molecular emission around T Tau

The first complete far-infrared spectrum of T Tau has been obtained with the LWS spectrometer on-board the Infrared Space Observatory, which detected strong emission from high-J (J=14-25) CO, para- and ortho-H2O and OH transitions over the wavelength range from 40 to 190 micron. Most of the observed molecular emission can be explained by a single emission region at T~300-900 K and n(H_2)~10^(5-6) cm^(-3),with a diameter of about 2-3 arcsec. This corresponds to a very compact region of 300-400 AU at the distance of 140 pc. An higher temperature component seems to be needed to explain the highest excitation CO and water lines. We derive a water abundance of 1-7x10^(-5) and an OH abundance of ~3x10^(-5) with respect to molecular hydrogen, implying water and OH enhancements by more than a factor of 10 with respect to the expected ambient gas abundance. The observed cooling in the various species amounts to 0.04 L(sun),comparable to the mechanical luminosity of the outflow, indicating that the stellar winds could be responsible of the line excitation through shocks. In order to explain the observed molecular cooling in T Tau in terms of C-type shock models, we hypothesise that the strong far-ultraviolet radiation field photodissociates water in favour of OH. This would explain the large overabundance of OH observed. The estimated relatively high density and compactness of the observed emission suggest that it originates from the shocks taking place at the base of the molecular outflow emission, in the region where the action of the stellar winds from the two stars of the binary system is important.

astro-ph↗