Parallel Sessions: ISM and Star Formation

1. Parallel Sessions: ISM and Star Formation I’ve been asked to report on the parallel sessions on the interstellar medium and star formation. For me the high point of the meeting was yesterday afternoon's session. It was kicked off by Philippe Andre who gave the most concise description I had heard of the direction that infrared and submillimeter astronomy should be heading: He asked, “What determines the Initial Mass Function, IMF? What generates pre-stellar cores? What are the time scales? Why and how do protostellar regions collapse? What makes them fragment? How is turbulence involved? The mass distribution of pre-stellar condensations resembles the IMF, which is at least in part determined by cloud fragmentation. But what generates pre-stellar condensations in the ISM? Philippe then described a program with Herschel and ALMA to at least partially answer these questions.

2. Pre-Stellar Cores The next speaker was Ted Bergin, who sketched the nature of pre-stellar cores—interstellar cloud cores not yet complicated by the burdens of collapse and, therefore, sufficiently simple to be credibly modeled with means at hand today. B68 is such a cloud. Its column density can be judged, as Alves, Lada & Lada (2001) first showed, by its extinction of background sources. From this density profile and radiative transfer calculations, we can obtain a physico-chemical model. We assume that CO, CN and CS are tracers of the cloud’s surface composition, and that NH3 and N2H+ are tracers of the bulk of the cloud. We then model the chemical composition and physical characteristics of the cloud as a function of radial distance inward from the surface, provided we can correctly estimate the strength of the ambient surface radiation field, thus bounding physical conditions. The cloud is cold in its center, warm at the surface. Herschel and ALMA, as Ted promised, should fill in the “anatomical” details.

3. The Promise of Spectroscopy For a long time, we have been telling ourselves that spectroscopy will lead to greater insight. Pepe Cernicharo showed us how true this can be. Using archival ISO data he showed us how he and his colleagues extracted perhaps the most beautiful FIR /SMM astronomical spectrum I have seen (Gonzales-Alfonso et al., ApJ 613, 247, 2004). It portrays the ultraluminous infrared galaxy Arp 220 in the lines of H2O, OH, NH, CH, [C II], NH3 and [O I], in the wavelength range 50 – 180 m. This shows how much more can be achieved if one uses interactive techniques and knows how to proceed spectroscopically. None of the machines we are constructing are simple. This makes it difficult to extract the most complete information from raw data; and this is likely to remain true both of Herschel and ALMA. Pepe has shown us what we will need to do to take full advantage of these two missions.

4. Data Banks Pepe, Eric Herbst and Karl Menten all stressed the needs for greatly enlarged spectroscopic data banks. We need to know atomic, molecular and ionic level structures, bond strengths, and set up compendia of collision cross sections and reaction rates with likely ambient constituents. Eric stressed that these need to be available for insertion into plausible chemical models that might take the form of master equations involving gas/grain-surface reactions, or Monte Carlo models, and take into account relevant hydrodynamics and radiative transfer. None of this is simple – but that’s the way it is! Fortunately, as the posters showed, a number of groups are already setting up various data banks (Dubernet et al., Kerschbaum et al., Wakelam et al.), while others are providing urgently needed laboratory data (Bruneleau, et al., Chihara, et al., Hornekaer, et al., Koike et al., and Suto, et al.)

5. Laboratory Studies A lack of laboratory studies is likely to eventually impede interpretation of data that will become available with Herschel and ALMA. Though many colleagues, notably Xander Tielens, have been actively seeking a greater community-wide effort, much remains to be done. It is, therefore, gratifying that Kerschbaum’s group in Vienna has been conducting laboratory studies of likely solids of astrophysical interest, at low temperatures, in the far-infrared, where we know little. Forsterite, fayalite, frozen methanol, carbonates, and graphite, all have absorption/ emission features in the Herschel/PACS spectral band, providing us with new analytical means for estimating ambient physical and chemical conditions. Wil van Breugel spoke of experiments he is conducting with high energy particle bombardment. These suggest that interstellar cosmic rays are responsible for amorphosizing originally crystalline grains.

6. Theoretical Chemistry Eric Herbst gave a beautiful talk on progress in chemistry, emphasizing how much remains to be done if we are to derive full benefit from the funds expended on Herschel and ALMA. Some chemical reactions and physical characteristics are too difficult to measure directly and need to be calculated. The posters showed considerable efforts underway by Faure et al., Juvela and Padoan, Giuliano et al., Morata and Herbst, Giacomo et al., Pulecka et al., Rapacioli et al., Staeuber et al., Talbi and Chandler, Vaidya and Anandarao, Wickramasinghe and Wickramasinghe, and Woitke. Unfortunately, there still is insufficient funding for both laboratory studies and theoretical efforts, and we are likely to end up with observations on Herschel, SOFIA and ALMA that we are unable to adequately place in context.

7. A number of speakers, and several posters spoke of the. fractionation of deuterated species, H2D+, D2H+, N2D+, HDO, D2CO, CH2DO, CD2OH, ND, . . . Poster by Lis et al., and Vastel et al. showed that, by now, ND2H and ND3 have both been detected, in abundance, though the precise ratio of the two is not yet firmly established. Presumably this will happen soon. The fractionation is attributed to the reaction H3+ + HD -> H2D+ + H2 where the H3+ is believed to arise in the diffuse interstellar medium through the action of cosmic rays. By successive deuteration molecular species like ND3 come into being. The expected destruction of H3+ by CO should also be checked observationally. With as much attention as has been paid to deuteration in recent years, the reactions involved may soon be reliably understood. Deuterated Species

8. Beautiful Images and Spectra We saw some beautiful images, that drive home with astounding clarity how complex some star-forming regions are, and yet how much more we know for seeing the pictures of them. Most of the images came from the recent first release of Spitzer data, which Tom Soifer presented. It is hard to over-estimate how much we will learn not only about Galactic star formation from these pictures, but also the closely related processes in nearby galaxies, and in the giant mergers that abound at red shifts z~1 to 2. The spectra that Spitzer is providing, with the discovery of new PAH features, and the sensitivity to reach out to great distances across the Universe should also contribute to our understanding of the local interstellar medium and how it gives rise to stars.

9. Multispecies Spectra of Different Clouds or portions of Clouds We also saw beautiful spectra of individual regions of complex clouds, showing the emission and/or absorption features of different chemical species. This is clearly a sensible approach. However, we should be prepared to accept that such multispecies investigations will have to be applied to many thousands of different regions, before we see a significant return. These spectra are usually going to be difficult to disentangle, since any given line of sight is likely to cut through a region that is just as complex along that sight-line as along any line that would transversely cut across the maps we now see Spitzer producing.

10. Ake Hjalmarson showed us a number of recently compiled submillimeter spectra obtained with the Odin satellite. Odin is a 1.1 meter telescope and, like the smaller SWAS telescope that preceded it, it has carried out pioneering studies of water vapor and other species at kilometer-per-second spectral resolving powers. Unlike SWAS, Odin has also been able to obtain spectra over sizeable wavelength ranges. Ake showed us a portion of such a spectrum of the Orion region from ~550 to 557.5 GHz. He was disappointed that he was able to explain every one of the many spectral features the plot exhibits. He had hoped for new features that might reveal more. However, as Karl Menten pointed out in his talk, the submillimeter range is expected to swamp us with unexplainable data from Herschel/HIFI and ALMA. So, perhaps we should be happy to have found at least one spectrum we understand. New Odin Results

11. The Outflow from AGB Stars (I) This morning we had exciting talks by Thibaut Lebertre, Hans Olofsson and Tom Millar on the outflow from AGB stars and their successors, the proto-planetary nebulae. Among AGB stars only IRC +10216 currently exhibits a useful number of spectral features to permit detailed analysis of physical conditions in the outflow. Herschel and ALMA are likely to provide us with a much longer list of chemical species, with sufficient numbers of individual features for many of these to permit reasonable modeling. With better data, however, we will also need greatly improved analytical tools so that the outflow may be properly understood. Current mass loss estimates for many of these stars are wildly divergent, depend on a wide variety of different assumptions, and prevent us from satisfactorily estimating the rate at which heavy elements from the outflows are enriching the Milky Way interstellar medium.

12. The Outflow from AGB Stars (II) In this context, I’d like to call attention to a striking poster by Pulecka, Schmidt and Szerba. Until about five years ago, it was acceptable to fit the spectrum of a dust-shrouded evolved star by postulating the superposition of a variety of dust species, PAHs, very small grains, and big grains, at different temperatures and radial distances from the star. Though this procedure provided good-looking fits, it often violated laws of physics. Only slightly better, was a fit that took radiative transfer into account, but postulated a distribution of circumstellar dust that again was arbitrary except in yielding a good fit. About five years ago, Moshe Elitzur and coworkers provided a DUSTY code to couple radiative transfer and dynamics, thus providing a physically more plausible outflow. Pulecka et al. have now added two more steps. They take into account both gravity and chemical reactions in the star’s atmosphere and outflow. This, finally, is a credible model, and should yield correspondingly satisfactory insights.

13. High-Mass Star Formation Frederic Schuller and Vincent Minnier spoke about very different approaches for finding sites of high-mass star formation -- about which we still know comparatively little. Minnier’s talk reminded me of an old paper,”Infrared and Radio Appearance of Cocoon Stars,” ApJ 148, 443, 1967, that Kris Davidson and I wrote nearly forty years ago. At the time, we were just beginning to fly liquid helium-cooled telescopes with far-infrared detectors on rockets and wondering what a newborn star might look like before it had blown away the cocoon of dust within which it was born? Kris, then a first year graduate student at Cornell, stopped by my office one day, and we decided we’d look into it. Clearly, the star would heat the dust and we’d see the far infrared radiation. And Kris figured that a sufficiently massive star would ionize the gas immediately around it and emit free-free radiation. So we wrote this up and sent it off. It is nice to see that a FIR source associated with an ultra-compact H II region still fits the prescription today. Even simple ideas often take many decades to check out.

14. Summary The lessons I will be taking home with me from this workshop are that the incredibly powerful observatories we are now building and will soon be using, are going to yield such enormous amounts of data, that matters will become considerably more complex before we can pick our way through the trove of data and re-establish a level of simplicity. It will be hard work but also exhilarating. And when it is all over, we’ll understand the interstellar medium and star formation in considerably greater detail than we do today. Speakers after speaker seemed to agree that ALMA and Herschel will force us to be far more systematic in approaching our handling of data than ever before. They will make us set up vastly more comprehensive compendia of chemical reaction chains and other properties of matter, and will require us to install complex computational modeling tools. Herschel and ALMA will drastically change how we work!