R. Chini

A survey for circumstellar disks around young stellar objects

Results are presented from a survey for 1.3 mm radiation toward 86 stars in the Taurus-Auriga dark clouds, including classical T Tauri stars, stars in T associations, and a few weak emission-line stars or naked T Tauri stars. The results show that 42 percent of the stars have detectable emission from small particles. The aggregate particle masses are found to be between 0.00001 and 0.01 solar mass, suggesting total disk masses between 0.001 and 1 solar mass. For several source between 1.3 and 2.7 mm, the spectral indices indicate that the particle emissivities are weaker functions of frequency, compared to the usual case of interstellar grains. Particle growth via adhesion in the dense disks is proposed to explain this result. The results show that disks more massive than the minimum mass of the protosolar system commonly accompany the birth of solar-mass stars, indicating that planetary systems are probably common in the Galaxy. 59 refs.

SIMBA survey of southern high-mass star forming regions. I. Physical parameters of the 1.2-mm/IRAS sources

We report the results of a 1.2 mm continuum emission survey toward 146 IRAS sources thought to harbour high- mass star forming regions. The sources have FIR colors typical of UCHII regions and were detected in the CS(2 → 1) line survey of Bronfman et al. (1996). Regions of 15 � × 10 � , centered on each IRAS source, were mapped with an angular resolution of ∼24 �� , using the SIMBA array on the SEST telescope. 1.2 mm emission was detected toward all IRAS sources. We find that the dust cores associated with these sources have typical sizes of 0.4 pc and masses of 5 × 10 3 M� . Dust temperatures and luminosities, derived from the SED, are typically 32 K and 2.3 × 10 5 L� .

Infrared to millimetre photometry of ultra-luminous IR galaxies ?;?? : New evidence favouring a 3{stage dust model

Infrared to millimetre spectral energy distributions (SEDs) have been obtained for 41 bright ultra- luminous infrared galaxies (ULIRGs). The observations were carried out with ISOPHOT between 10 and 200 m and supplemented for 16 sources with JCMT/SCUBA at 450 and 850 m and with SEST at 1.3 mm. In addition, seven sources were observed at 1.2 and 2.2 m with the 2.2 m telescope on Calar Alto. These new SEDs represent the most complete set of infrared photometric templates obtained so far on ULIRGs in the local universe. The SEDs peak at 60{100 m and show often a quite shallow Rayleigh-Jeans tail. Fits with one single modied blackbody yield a high FIR opacity and small dust emissivity exponent T >30 K) to cold (30 K >T >10 K) temperatures. This provides evidence for two dust stages, the cool starburst dominated one and the cold cirrus-like one. The third stage with several hundred Kelvin warm dust is identied in the AGN dominated ULIRGs, showing up as a NIR-MIR power- law flux increase. While AGNs and SBs appear indistinguishable at FIR and submm wavelengths, they dier in the NIR-MIR. This suggests that the cool FIR emitting dust is not related to the AGN, and that the AGN only powers the warm and hot dust. In comparison with optical and MIR spectroscopy, a criterion based on the SED shapes and the NIR colours is established to reveal AGNs among ULIRGs. Also the possibility of recognising evolutionary trends among the ULIRGs via the relative amounts of cold, cool and warm dust components is investigated.

A spectroscopic survey on the multiplicity of high-mass stars

The formation of stars above about 20 M⊙ and their apparently high multiplicity remain heavily debated subjects in astrophysics. We have performed a vast high-resolution radial velocity spectroscopic survey of about 250 O- and 540 B-type stars in the southern Milky Way which indicates that the majority of stars (>82 per cent) with masses above 16 M⊙ form close binary systems while this fraction rapidly drops to 20 per cent for stars of 3 M⊙. The binary fractions of O-type stars among different environment classes are: clusters (72 ± 13 per cent), associations (73 ± 8 per cent), field (43 ± 13 per cent) and runaways (69 ± 11 per cent). The high frequency of close pairs with components of similar mass argues in favour of a multiplicity originating from the formation process rather than from a tidal capture in a dense cluster. The high binary frequency of runaway O stars that we found in our survey (69 per cent compared to 19–26 per cent in previous surveys) points to the importance of ejection from young star clusters and thus supports the competitive accretion scenario.

Size and properties of the narrow-line region in Seyfert-2 galaxies from spatially-resolved optical spectroscopy ?

Context. While [Oiii] narrow-band imaging is commonly used to measure the size of the narrow-line regions (NLRs) in active galactic nuclei (AGNs), it can be contaminated by emission from surrounding starbursts. Recently, we have shown that long-slit spectroscopy provides a valuable alternative approach to probe the size in terms of AGN photoionisation. Moreover, several parameters of the NLR can be directly accessed. Aims. We here apply the same methods developed and described for the Seyfert-2 galaxy NGC 1386 to study the NLR of five other Seyfert-2 galaxies by using high-sensitivity spatially-resolved optical spectroscopy obtained at the VLT and the NTT. Methods. We probe the AGN-photoionisation of the NLR and thus, its “real” size using diagnostic line-ratio diagrams. We derive physical properties of the NLR such as reddening, ionisation parameter, electron density, and velocity as a function of distance from the nucleus. Results. For NGC 5643, the diagnostic diagrams unveil a similar transition between line ratios falling in the AGN regime and those typical for Hii regions as found for NGC 1386, thus determining the size of the NLR. For the other four objects, all measured line ratios fall in the AGN regime. In almost all cases, both electron density and ionisation parameter decrease with radius. Deviations from this general behaviour (such as a secondary peak) seen in both the ionisation parameter and electron density can be interpreted as signs of shocks from the interaction of a radio jet and the NLR gas. In several objects, the gaseous velocity distribution is characteristic for rotational motion in an (inclined) emission-line disk in the centre. We compare our results to those of NGC 1386 and show that the latter can be considered as prototypical also for this larger sample. We discuss our findings in detail for each object.

The formation of a massive protostar through the disk accretion of gas.

The formation of low-mass stars like our Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above ten solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via accretion through a disk. Given this uncertainty and the fact that most massive stars are born in dense clusters, it was suggested that high-mass stars are the result of the runaway merging of intermediate-mass stars. Here we report observations that clearly show a massive star being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and the accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star.

Mm/Submm images of Herbig{Haro energy sources and candidate protostars

We present 450, 850 m and 1300 m images of regions with embedded candidate protostars. Some of them are associated with HH objects (HH 7-11, HH 1-2, HH 147, HH 111, HH 108) and have been previously iden- tied in the course of a 1300 m survey. Other regions were taken from the IRAS PSC (04239+2436, 04368+2557, 20050+2720, 20386+6751, 22134+5834, 23011+6126). The new mm/submm images show the detailed structure of the regions some of which contain new compact sources as well as extended emission features. The inferred mm/submm fluxes are combined with IRAS data in order to derive the temperature of the associated dust, its mass and the re{radiated luminosity. Taking the ratio of FIR{to{submm luminosity as an indicator for the evolu- tionary stage, we nd that 15 out of 17 sources have LFIR=Lsmm < 200, indicating that most objects are probably genuine protostars. For the rst time, we detect dust emission associated with HH objects themselves, H2 and CO flows which we interpret as density enhancements swept up by the ejected material.

Discovery of four new massive and dense cold cores

We report the identification, from a 1.2 mm dust continuum emission survey toward massive star-forming regions, of four strong 1.2 mm sources without counterparts at mid-infrared (Midcourse Space Experiment [MSX]) and far-infrared (IRAS) wavelengths. They have radii in the range 0.2-0.3 pc, dust temperatures ≤17 K, masses in the range 4 × 102-2 × 103 M☉, and densities of ~2 × 105 cm-3. We suggest that these objects are massive and dense cold cores that will eventually collapse to form high-mass stars.

The Stellar Population of M17

The stellar content of M17 has been investigated by multicolor photometry and spectroscopy. Various independent estimates yield a distance of 2.1 ± 0.2 kpc. The ratio of total-to-selective extinction is R = 3.9. Within a projected area of 3.6 × 3.7 pc, there are several thousand stars. About 74% of them show infrared excess suggesting the presence of dense circumstellar material; the excess frequency is higher for fainter stars. The number of spectroscopically classified exciting stars could be enlarged from 13 to 46. The two central O4 stars are both spectroscopic binaries; multiplicity of other early O-type stars could also be established, increasing the number of high-mass stars even further. Our data suggest at least two episodes of star formation: There are about 500 ZAMS sources (2 < AV < 7)—among them many spectroscopically classified OB stars and a significant fraction of lower mass sources with infrared excess (~25%) and X-ray emission (~6%). About 3350 heavily reddened sources with 10 < AV < 40) are most likely deeply embedded pre-main-sequence objects with an age of less than 5 × 105 yr. This group contains about 47% sources with infrared excess and 12% X-ray emitters. Cluster members later than about A0 have not yet reached the main sequence. In addition, a group of 647 protostellar candidates (1.5 < K − L < 6.9) has been detected in the cluster center as well as in the northern and southwestern bar. This population of accreting protostars argues in favor of ongoing star formation triggered by the central O stars in M17.

Dust emission from 3C radio galaxies and quasars: New ISO observations favour the unified scheme

In order to test the unied scheme for luminous radio galaxies and quasars we observed 10 galaxy/quasar pairs from the 3CR catalogue with ISOPHOT at infrared wavelengths between 5 and 180 m. Each pair was selected such that both the 178 MHz luminosity and the redshift match as close as possible between the radio galaxy and the quasar in order to minimize eects of cosmic evolution. 13 of the 20 sources were detected in at least one waveband. 12 sources show clear evidence of a thermal bump at FIR wavelength, while in the remaining 7 sources the upper limits are still compatible with the presence of luminous dust emission. In agreement with the predictions of the unied scheme, the quasars and galaxies in our sample cannot be distinguished by their observed mid- and far-infrared properties. This is in contrast to the ndings on the basis of the IRAS scans which indicated that radio galaxies radiate signicantly less mid- to far-infrared emission than quasars. However, the IRAS samples are dominated by low-redshift sources ( z 0:8). At lower redshifts ( z< 0:5), however, some of the lobe-dominated FRII radio galaxies contain active nuclei which emit less UV-optical continuum than the quasars of similar radio power. As this division is mainly a function of redshift and less one of absolute radio power, we suggest that it is caused by the evolution of the nuclear fueling rate with cosmic epoch. In order to quantify the deviation from the purely aspect-dependent unied scheme at low redshifts a larger fraction of 3C radio galaxies has to be observed at mid- to far-infrared wavelengths with sensitivities which suce to yield secure detections rather than upper limits.