Pub. Astron. Sex:. Pacific, Volume 86, October 1974 on the NATURE OF

Pub. Astron. Sex:. Pacific, Volume 86, October 1974

ON THE NATURE OF THE SMALL DARK GLOBULES IN THE ROSETTE NEBULA*

G. H. HERBIG Lick Observatory, Board of Studies in Astronomy and Astrophysics University of California, Santa Cruz Received 6 July 1974

The small dark globules seen against the nebulosity in the northwest quadrant of NGC 2237-2244 are shown on a 120-inch direct photograph to have an elongated, tear-drop form with the symmetry axes and sharper edges directed toward the central star cluster. This orientation is shared by the well-known elephant trunk structure, which in general lies farther from the center than these isolated dark spots. Examples of globules still connected with large dark masses by dust filaments are also present. It is suggested that these globules represent a late stage in the pinching-off and dissipation of elephant trunks as the central cavity of the Η ii region expands into the peripheral dust clouds, and that these globules are not protostars. It is estimated that the age of a typical small isolated globule in this region of NGC 2237-2244 is of the order of 104 years. Key words: globules — dark nebulae — Η π regions

If a star is formed by the contraction of a large, 120-inch reflector (13" mm-1), however, a num- cool, dusty interstellar cloud to stellar dimen- ber of interesting structural details become ap- sions, then in the process it must pass through the parent. (A) Many of these small features, down intermediate radius of about 104 a.u. as a highly to about 3" across, clearly are seen to have an opaque condensation of very low surface tempera- arrowhead- or teardrop-shaped outline. Still ture: a "protostarThe purpose of this note is smaller dark spots are present but at this resolu- to suggest, however, that one of the best-known tion their true shapes are not discernible. Fur- groups of very small dark nebulae having dimen- thermore, (B) the orientation of the symmetry sions of the order of 104 a.u. which are often cited axes of these 'teardrops' is in general parallel to in this connection are in fact not protostars, but that of the elephant trunks or pennisulae which can better be accounted for in another way. protrude from the extensive masses and streamers This note also is intended to elaborate upon a of dust in the vicinity. Around an arc of 90° with remark I made several years ago (Herbig 1970) respect to the center of the Rosette Nebula, the to the effect that "the small dark globules that are symmetry axes of both teardrops and elephant seen against some Η π regions . . . have [ no trunks all are directed inward toward the central demonstrable] connection with star formation." cluster of bright stars. (C) In most cases, the Attention was first called to the remarkable sharp edge of the teardrop is toward the cluster, structure of the dark markings in the northwest and the diffuse or extended edge lies in the oppo- quadrant of the Rosette Nebula NGC 2237-44f site or shadowed direction. by Minkowski (1949), who illustrated his paper Plate I is an enlargement of part of a 50-minute with the now-familiar 48-inch Schmidt photo- red exposure with the 120-inch telescope, show- graphs of the Nebula. At that plate scale (67" ing the structure of the dark markings in the most mm-1), the shape of the smaller dark spots was complex region of the northwest quadrant. Plate not obvious. At the prime-focus scale of the Lick II shows the entire plate, with arrows drawn to indicate the orientation of all the teardrops in ''Lick Observatory Bulletin, No. 672. that area whose orientation can be assigned with f The star cluster is NGC 2244, while the Rosette nebu- some confidence. The alignment of elephant losity is NGC 2246. It is not clear whether NGC 2237 is trunks in the same sense is striking. Spectral part of the nebulosity or a duplicate entry for NGC 2246 with slightly erroneous coordinates. The traditional de- types identify three of the bright early-type mem- signation is used here. bers of NGC 2244 in the comer of the photograph; 604

HD 46223 (04) and HD 46202 (09 V) lie just out- the vicinity swept clear of interstellar material. side the area shown. One would estimate that A good example of such a cluster is NGC 2362. the photocenter of the O-type stars lies not far It is proposed that the observed properties (A) from HD 46150, the OS star at the edge of Plate through (D), above, of the dark material in Plates II. The arrows suggest a convergence slightly to I and II can be understood if the dust in that re- the west (right) of that star. Attention is called gion is now in stage (3). The small isolated tear- to the further significant fact that (D) most of the drops lie nearest the cavity center; they are ap- isolated dark teardrops lie in projection nearer parently the pinched-off remains of elephant the central star cluster than do the nearby ele- trunks and are soon to disappear. The teardrops phant trunks and larger masses of dark material. still connected with the main masses of dust by a I interpret this situation as one expected to vestigial tentacle then represent the immediate arise at a relatively late stage in the evolution of progenitors, a late stage in the dissolution of an the central star cluster and its parent interstellar elephant trunk. The still-unbroken trunks, pro- cloud. The time sequence of events is envisaged truding like cumulus thunderheads from the in- as follows. ner edges of large dust masses, represent a still 1. The cluster NGC 2244 must have formed earlier stage. from the dense central region of a massive Η ι A crude estimate of the lifetime of the dust cloud. An Η ι region is postulated because there structures can be made if one assumes that the is no evidence of evolved high-luminosity mem- dust continues to disappear as the cavity expands bers of an earlier generation in the neighborhood. at the average past rate. The present radial sepa- The Trapezium region of the Orion Nebula, where ration of the smallest isolated dark teardrops a cluster of young stars lies at a density maximum from the large dark masses ranges from 25" to in the gas, is an example of how the NGC 2244 50" in this quadrant of the Nebula. The cavity region may have looked several 105 years ago. boundary will move across this distance in 8 X 2. The cavity now surrounding NGC 2244 103 to 16 X 103 years. On the same grounds, the appears to be a real one. The residual inter- lifetime remaining to a cloud 3" in diameter is stellar material could have been driven out of about 103 years. Short as it is, this interval is too this region by stellar winds (Mathews 1966) or by long for teardrop shrinkage to be detectable over radiation pressure on dust (Mathews 1967). the few decades that photographs of the neces- Smith (1973) has observed that the [Om] emis- sary resolution are available. sion lines are double with a splitting of 41 km A closely similar sequence of evolution for s-1 near the cavity center. The cavity radius at bright rims has been proposed by Pottasch (1958a, the present time is about 9'5, which is about b, 1965), and interpreted by him in terms of 9 X 105 a.u. at the distance of 1.6 kpc (Becker evaporation of ionized gas from the surface of and Fenkart 1971). Thus the time for the moving neutral condensations exposed to a strong direc- [Om] to traverse the cavity radius is 2 X 105 tional radiation field. Few of the teardrops in years.* We suppose that the cavity is now ex- Plates I and II have bright rims detectable on the panding at that rate, gas and dust being driven 120-inch plate,f but some of the elephant trunks before it. are slightly luminous on their inner edges. 3. Concentrations of gas must in this way be It is not at all clear how the dust concentrations dispersed radially, with the process now being "disappear". The teardrop shapes do suggest underway around the cavity boundary. If any dissipation or evaporation in a radial wind, but untouched concentrations of the original dust in fact the present observations do not rule out remain, they will be found outside the boundary. the alternate possibility that following the break- 4. The final stage will be one in which the up of the trunks, the small discrete clouds simply young star cluster —by then slightly evolved — vanish upon contraction to below the resolution remains, with star formation having ceased and of present photographs. If one is inclined to *This value is compatible with the prediction of a model f There is one exception, a small dark globule encircled by Mathews (1967), namely that the age of the Nebula by a luminous ring. In projection, it lies well within the since ionization lies between 2 X 105 years (for uniform cavity. It can be seen in Plate II, 90 mm above and density at the beginning) and 6 X 105 years. 18 mm to the right of the lower-left corner.

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PLATE II The northwest quadrant of the Rosette Nebula, from the same negative as Plate 1. Arrows show the pointing direction of those teardrops whose orientation is apparent on the original plate. Spectral types of several early-type members of the cluster NGC 2244 are indicated. A number of streaks extending perpendicularly from the right and left margins are caused by reflections in the plateholder from bright stars just outside the field.

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 608 G. H. HERBIG think of such hypothetical condensations as the 1973) provide a starting point for star forma- seeds of future stars, however, a conflict with ob- tion. But my own opinion is that the "glob- servation arises. That is, if these condensations ules" which we can see optically cannot be iden- were massive enough to continue in their contrac- tified with that phase of stellar evolution. Star tion and ultimately to form stars, one would ex- formation must usually begin with the condensa- pect to see young stars continuing to appear tion of dense subclouds deep within the cool in- around the peripheral regions of the cluster, long teriors of large dark nebulae, and these very re- after the nebulosity had vanished. But this situa- quirements militate against their detection at tion has not been observed in nature: young stars optical wavelengths. As far as direct demon- of high and intermediate mass are invariably stration or even persuasive speculation is con- associated with dense nebulosity, and older ex- cerned, I must agree with van den Bergh (1972) amples of gas-free clusters (like NGC 2362) are that "there is at present no firm evidence that any not accompanied by haloes of very young mem- stars are formed from globules". bers. In fairness, on the other hand, one can say This publication has been supported in part nothing about the possibility that these hypothe- by NSF grant GU-3162. tical condensations are of low mass: in that case, embryo stars of a few tenths of a solar mass might REFERENCES be present in significant numbers, and their opti- Becker, W., and Fenkart, R. 1971, Asir, and Ap. Suppl. cal appearance would be delayed for some time. 4, 241. To summarize, it is suggested that the intri- Bergh, S. van den 1972, Vistas in Astronomy 13, 265. Bok, B. J. 1948, Centennial Symposia (Cambridge: Har- cate system of very small dark markings in the vard Observatory), p. 53. Rosette Nebula has no demonstrable direct con- Bok, B. J., and Cordwell, C. S. 1973, in Molecules in nection with star formation, and in the present the Galactic Environment, M. A. Gordon and L. E. state of our information surely should not be Snyder, eds. (New York: lohn Wiley & Sons, Inc.), considered as protostars. Rather this Nebula p. 54. Herbig, G. H. 1970, Évolution Stellaire Avant la Sé- appears to provide a snapshot of the interesting quence Príncipale, Mem. 8° Soc. Roy. Sei. Liège, process by which residual gas and dust in the 5th ser., 19, p. 13; Lick Contr. No. 302. neighborhood of a hot star is dissipated. No Mathews, W. G. 1966, Ap./. 144, 206. attempt is made here to provide a reasoned an- 1967, ibid. 147, 965. swer to the more general question of whether any Minkowski, R. 1949, Pub. A.S.P. 61, 151. Pottasch, S. 1958ö, Bull. Astr. Inst. Netherlands. 14, 29. of the variety of roundish dark "globules" and 19586, Rev. Mod. Phys. 30, 1053. Barnard objects that have been discussed by Bok 1965, Vistas in Astronomy β, 149. and his associates (Bok 1948; Bok and Cordwell Smith, M. G. 1973, Ap.J. 182, 111.