INTERSTELLAR MEDIUM AND STAR FORMATION

Involved IRA Scientists and Collaborators: J. Brand, F. Fontani

INTERSTELLAR MEDIUM

We carry out multi-line and continuum radio and (sub-)mm observations of molecular clouds across the Galaxy, with single-dish and interferometric telescopes. The main aims are to determine the physical and chemical properties of individual clouds, and to study Galaxy-wide phenomena such as abundance gradients and gas-to-dust ratios. We are also carrying out a search for the presence of pre-biotic molecules at the edge of the Galactic disk, in order to put constraints on the size of the Galactic Habitable Zone. Selected publications: Wouterloot et al. 2005; Brand et al. 2001a; Brand et al. 2001b.

STAR FORMATION

Massive stars (M>8 Mo) transfer large amounts of energy to the Interstellar Medium (ISM) through stellar winds, supernova explosions, and copious amounts of UV-radiation.  These processes can lead to the formation of new generations of stars, for instance in small molecular clouds that are inside or at the edges of supernova-created bubbles, or at the borders of expanding HII regions.  In collaboration with the observatories of Marseille, Grenoble, and Arcetri we have selected a number of HII regions where, based on their morphology, we expect to have a good chance to see this induced star formation process at work. Several of these regions have been investigated, and for a few of them we have been able to establish that the embedded star cluster at the border of the HII region has likely been formed by this process. A good example is shown in Figure 14 (taken from Zavagno et al.  2006).

Fig 14 - RCW79. Cold dust condensations seen in the mm-continuum mark the presence of dense swept-up gas at the borders of the ionized hydrogen region RCW79. The inset shows a NIR image of a cluster of stars, embedded in one of the larger condensations. Stars of this cluster excite an ultra compact HII region (from Zavagno et al. 2006).

Spontaneous (i.e. not triggered by external agents) star formation is studied in Bok Globules. These are relatively isolated nearby (typically a few 100 pc), small (typical diameter 0.7 pc) and  cold (10 K) molecular clouds, often with dense cores and embedded protostars (detected in the NIR).  Because of their small size and mass, they form low-mass stars in small numbers and therefore without the observational confusion that one encounters in regions like Ophiuchus or Orion. TNG (Telescopio Nazionale Galileo) observations were performed for a sample of globules in the NIR through narrow-band H_2 (tracing slow shocks) and [FeII] (excited in fast shocks) filters. The JCMT and the IRAM 30-m were used to observe the large-scale outflows and the dense core in which the Young Stellar Object (YSO) driving the outflow is embedded. One of the best-studied objects is globule CB230, in which a jet with two knots emanating from the YSO was detected. The jet is aligned with the large-scale outflow, detected in CO(3-2) (Figure 15). NIR spectra of the YSO and knots confirmed the presence of H_2 and [FeII] emission, and which also indicate the presence of a circumstellar disk around the YSO (in collaboration with Arcetri, TNG-La Palma, and JAC-Hawaii).

Fig 15 - Globule CB230. The contours show the red- and blue-shifted lobes of the root of the large-scale bipolar outflow driven by the embedded Young Stellar Object (YSO; indicated by CB230-A). The greyscale shows the [FeII] line emission of the jet originating from the YSO. Note that jet and large-scale flow have the same orientation, but that the molecular flow deviates to the West at larger distance from the YSO. This may be due to density inhomogeneities in the gas in the globule, or to precession of the jet. The jet is mono-polar; its counterpart, which would drive the red lobe of the large-scale outflow, is presumably obscured by the material around the YSO. This could happen if the flow axis is tilted with respect to the plane of the sky - Courtesy of J. Brand.

In the far-outer Galaxy (R > 16 kpc) the physical environment differs from that in the inner Galaxy, the effects of which may influence the formation of molecular clouds and the star formation process within them. To study the process of star formation and its end products in the outer parts of the galactic molecular disk, in collaboration with the Joint Astronomy Center (Hawaii) we are making a census of molecular clouds and star formation at the edge of the Galactic molecular disk. One of the final aims is to derive the IMF of the embedded clusters in order to establish whether it differs from the local IMF. As an example we show in Figure 16 and Figure 17 a false-colour image of the most distant star cluster found in the Galaxy so far, and its associated outflow (Brand & Wouterloot 2007).

Fig 16 - False-colour image (combination of the NIR J-, H-, and K-band images) of the star cluster associated with WB89-789 (IRAS06145+1455) (from Brand & Wouterloot. 2007).

Fig 17 - Map of the integrated CO(3-2) wing emission, showing the red-and blue- shifted lobes of the molecular outflow associated with WB89-789. The IRAS position is at offset (0,0); the triangle marks the peak of the 850 microm dust continuum emission. The underlying image is the K-band frame of the region. Contours (black) are drawn to highlight two extended features near offsets (10", -3") and (-16", 4"), seen as red spots in Fig. 16, that might mark the spots where the outflow impacts with the ambient medium (from Brand & Wouterloot 2007).

This cluster lies embedded in a molecular cloud at the very edge of the Galaxy (R ~19 kpc). For more details see e.g.: Wouterloot & Brand 1989; Wouterloot et al. 1990; Brand & Wouterloot al. 1995; Brand et al. 2001.

EARLY STAGES OF MASSIVE STAR FORMATION

Pre-stellar cores are cold and dense molecular condensations from which protostars are born via gravitational collapse. We search for and study massive pre-stellar cores close to some well-know high-mass protostellar objects (HMPOs), using the same investigative techniques successful in the identification of low-mass pre--stellar cores, namely high values of the column density ratio N(N^2 D^+)/N(N_2 H^+) and high values of the CO depletion factor. In collaboration with the Smithsonian Astrophysical Observatory, the Arcetri Observatory and the University of Leiden, we have observed with the IRAM-30m telescope 10 HMPOs in several rotational lines of N_2 D^+, N_2 H^+ and C^17 O, and in the sub-mm continuum with the JCMT. We have derived values of the deuterium fractionation 3 orders of magnitude higher than the cosmic value, while the observed CO abundance is on average 3 times smaller than the theoretical value (Fontani et al. 2006).
We are extending this study to a larger sample of objects, using the JCMT, while individual objects are studied in detail with the SMA and PdB interferometers.

Hot Molecular Cores (HMCs) are hot, dense and compact molecular condensations where early-type stars have recently been formed. Their spectra show high abundances of saturated species (e.g. H_2 O, NH_3, CH_3 OH) and complex organic H-rich molecules. In some of them a spatial separation has been noted between molecules containing Nitrogen and those containing Oxygen. Chemical models explain this as being due to the different physical properties of the two regions, and predict a relation between the core evolutionary stage and the abundance ratio of pairs of molecular species like ethyl-cyanide (C_2 H_5 CN) and vinyl-cyanide (C_2 H_3 CN). Therefore, the relative abundance of these species can be used as a 'chemical clock' to estimate HMC ages. In collaboration with the Steward Observatory, and with Arcetri, we have observed 12 well-known HMCs in two N-bearing molecules (C_2 H_5 CN and C_2 H_3 CN), and two O-bearing molecules (CH_3 O CH_3 and HCOOCH_3) with the IRAM-30m telescope. By comparing the abundance ratio of the parent/daughter pair C_2 H_5 CN/C_2 H_3 CN with the predictions of chemical models, we have derived that the ages of our HMCs range between 3.7 and 5.9 x 10^{4} yrs (Fontani et al. 2007).

MASERS IN REGIONS OF STAR FORMATION AND IN CIRCUMSTELLAR ENVELOPES

Water masers are associated with the earliest stages of the formation of (especially massive) stars. Maser emission from star forming regions is highly variable both in intensity and in velocity thus long-term variability monitoring is important. In collaboration with Arcetri a monitoring survey of 45 star forming regions, associated  with IRAS sources with a luminosity between 20 and 2x10^{6} Lo is carried out with the Medicina 32-m telescope.  With a time coverage of 20 years, this is the most extensive data base of its sort in existence, together with that of the (Russian) Pushchino Observatory. An example of the available data is presented in Figure 18 which shows the behaviour of the flux density (colour scale) of the maser associated with L1204-G as a function of velocity and time, over a period of about 6200 days. Details in: Brand et al. 2003, Felli et al. 2007.

Fig 18 - Medicina water maser observations of L1204-G. Flux density is shown in colour-scale as a function of velocity and time. The time range shown is Dec. 1989 to Feb. 2007. The dashed line indicates the systemic velocity of the star-forming region. From diagrams like this one can easily identify bursts and accelerating or decelerating maser components   - Courtesy of J. Brand.

In collaboration with Hamburg and Onsala, we are carrying out a long-term monitoring program of water masers in the circumstellar envelopes of about 20 late-type stars (Mira's, supergiants, OH/IR stars, semi-regular variables). This program too has been active for about 20 years at the 32-m telescope at Medicina, and has simultaneously run for several years at the Effelsberg 100-m dish. We combine the single-dish data with interferometric (VLA) observations to derive the distribution and evolution of individual maser spots in the stellar envelopes.  As an example we show in Figure 19 the distribution of maser spots in the circumstellar shell of RX Boo. To find out more, see e.g.: Engels et al. 1997.

Fig. 19 - RX Boo. All the maser components found with the VLA plotted on the sky.  Each component is represented by a symbol and a circle whose diameter is logarithmically proportional to the flux density of the component.  The epochs are represented by different symbols: 1990 February by small circles; 1990 June by asterisks; 1991 October by plus signs; 1992 December by crosses. The circles around the components are colour coded according to the line-of-sight velocity of the component, i.e. the `radial velocity' minus the stellar systemic velocity (+1 km/s). A circle (dashed) has been fitted to the components on the arc and the origin has been moved to the center of this circle. The filled dark red circle at the origin symbolizes the central star with a diameter of 19 mas - Courtesy of J. Brand.