Astronomers announced today that they have found important clues that may lead to a better understanding of how their birth place affects the lives of stars. In a report to the American Astronomical Society, meeting in Washington DC, Ms. J.S. Kim and Prof. F.M. Walter (Stony Brook University, Stony Brook, NY) and Dr. S.J. Wolk (Harvard Smithsonian Center for Astrophysics, Cambridge, MA) eported that massive hot stars appear to have a strong influence on the formation of low mass stars, and on the survival of protoplanetary dust disks. They have studied one of the few examples near the Earth where astronomers can observe the destruction of stellar birth clouds by energetic stars, and learned that when young stars like our Sun form under such environment, they may not be able to form or keep circumstellar disks.
Our Sun formed about 4.6 billion years ago from a cloud of interstellar gas and dust. We do not know whether the Sun formed in a fairly benign environment (like today's Taurus clouds), or within the harsher conditions of a massive stellar association, like the Orion Nebula, replete with ionizing radiation, strong stellar winds, and exploding stars. Eighteen orbits around the Galaxy have erased all evidence of the Sun's birthplace, but clues may reside in the facts that, unlike most stars, the Sun is not in a binary star system, that the Sun has a planetary system (with a massive planet far from the Sun, unlike the currently known extrasolar planetary systems), and in the pattern of extinct radioactive elements found in meteorites. Characterization of the products of star formation in diverse environments may help us to interpret these clues and deduce under what conditions our Sun and its planets were born.
Kim, Walter, and Wolk are surveying the star forming regions associated with cometary globules (CGs) and other areas in the Gum nebula (Figure 1). The Gum nebula, discovered by Dr. Colin Gum in 1952, is a 36 degree diameter shell of gas and dust surrounding a hot bubble of ionized gas (an HII region). Found in the Puppis and Vela constellations in the southern sky, the nebula is the largest HII region visible in the sky, in part because it really is large, and in part because it is fairly close to the Sun, at a distance of about 1500 light years. The nebula surrounds, and may be heated by, the Vela supernova remant, the massive hot stars zeta Puppis and gamma2 Velorum, and many other hot stars. More than 30 CGs lie around a bright shell in the Gum nebula. The CGs are the remnants of dense cores of molecular clouds that are being blown away by the strong winds from massive hot stars.
With its range of environments, the Gum nebula offers snapshots of different episodes of star formation frozen in time. To study the effects of the different environments, Kim, Walter, and Wolk observed a number of CGs at X-ray, optical, and near-infrared wavelengths. In the mid 1990s Walter and Wolk used the ROSAT X-ray observatory to look for young stars near CGs. Young stars are brighter emitters of X-rays than are older stars, because they rotate more rapidly and have stronger magnetic fields. They identified about 120 possible young stars. They and Ms. Kim then obtained optical and near-infrared images using the 1.5 and 0.9 meter telescopes at the National Science Foundation's Cerro Tololo Inter-American Observatory (CTIO) during 1996-2000. The optical images can be used to identify young stars from their colors to much fainter levels than possible with the X-ray images; the near-infrared images can be used to determine what stars have circumstellar or protoplanetary dust disks.
The CG30/31/38 complex consists of 7 CGs (Figure 2). The head-tail geometry resembles that of comets, hence their name. The tails are directed away from the Vela supernova remnant and zeta Puppis. The tails of CG30 and CG38 are stretched toward upper-right, but the direction of the tail of CG31 is different. The tail directions suggests that these CGs must have been affected by zeta Puppis within the past million years. The X-ray-emitting young stars are found under the head of CG31. ``It is likely that these stars were born about a million years ago, in a small dust cloud. We can see the stars now that the dust cloud has been blown away by the winds from the hot stars'', Kim said. None of these young stars appear to have protoplanetary dust disks. Walter explained that ``As the natal dust cloud dissipates, so does the source of the material that feeds the circumstellar disks. These disks disappear quickly, either by falling into the star or by forming planets. Thus high mass stars terminate low mass star formation by starving the disks. This affects the final mass of the star and the nature of its planetary system. Perhaps these young stars either do not have planets, or have only small planets, because massive Jupiter-like planets may take a long time to form.''
CG30 reveals a different situation. As in CG31, the head is surrounded by X-ray emitting stars, but these show signs of protoplanetary disks. Furthermore, a young stellar object is known to exist in the dense head of CG31. Kim suspects that here two episodes of star formation have occurred. The first generation of stars, seen in X-rays, was uncovered as the CG began to evaporate. But the process of blowing the globule away compresses it, and has triggered a second round of star formation. Here the hot massive stars have both terminated and initiated the formation of young stars. But why do the older stars still retain evidence of circumstellar dust disks? Maybe the disks of these stars were formed before zeta Puppis began to influence this region, or they are further away where the stellar wind would be weaker and have a harder time blowing away the material trapped by the gravity of the young star.
The diverse environments of the Gum nebula show that massive hot stars both trigger and terminate the formation of low mass stars. Perhaps depending on the proximity to the nearest massive hot star, some of these young low mass stars have circumstellar dust disks, while others do not. Since most low mass stars in the Galaxy appear to have formed in association with high mass stars, our Sun may have formed in a similar environment. Wolk cautions that due to the large size of the nebula, there is a considerable uncertainty in the distance to each globule or star forming region. The precise spatial relationships between the hot stars and the cometary globules, H~II regions and young stars are still poorly determined. Future observations, beginning with multi-object spectroscopy on the CTIO 4m telescope this spring, will go a long way towards clarifying these influences. But, as Kim said, ``It is clear that the final products of low mass star formation are greatly influenced by the environments within which they form, and that study of the many environments of the Gum nebula will help us figure out why stars and planets are as they are.''
Please contact J. Serena Kim for more information.
Jinyoung Serena Kim
Dept. of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, NY, 11794-3800 USA Office: room ESS 457A/443 (Tel:631-632-1176/9040).