Star Formation in the Isolated Molecular Cloud




Abstract



In the standard model of star formation, UV radiation from OB stars or supernovae shockwaves compress the cold material in giant molecular clouds (GMCs). The denser areas or cores collapse due to self-gravity and accrete material until there is enough pressure to ignite nucleosynthesis, marking the beginning of the proto-star phase. This model successfully describes the formation of large conglomerations of stars. However, the spatial distribution of stars in the universe cannot be fully explained if stars only form in large clusters. Thus, effective star formation in isolated molecular clouds, further from the massive complexes but most likely still induced by them, offers an explanation to the distribution of stars in our universe. The region around NGC1788 is such an isolated cloud. We are performing a multi-wavelength survey of the area around NGC1788 in order to characterize how star formation proceeds in isolated molecular clouds.



Background



NGC1788 is a reflection nebula about fifty parsecs to the west of the Orion (see Figure 1).1 LDN1616 is the dusty nebula encompassing NGC1788 and the surrounding area. Actually, LDN1616 merges with LDN1615 in a cometary shape (see Figure 2a & b). The exact distance to the nebula has many literature values ranging from 360 to 460 parsecs.2,3 The majority of the distance estimates are calculated by using the brightest star illuminating the cloud as a standard candle. In the case of NGC1788, that star is HD293815, a B9V star. The vast variation in distance estimates may be due to different calculations of the extinction, which stems from LDN1616/15. It is suspected that the star formation in NGC1788 has been induced most likely [by] the UV radiation from the past and/or present O stars in the Ori OB1 Association.3 This pressure may also be tearing the cloud apart (see Figure 3).2



Progress



We are cataloging what objects around NGC1788 appear in which wavelengths. We are nominally working with optical, infrared, and x-ray data but also with some radio data. We observe the region in optical [CTIO 0.9-m] and infrared [CTIO 1.5-m] ourselves and utilize collaborators’ data [ROSAT X-ray sources and ESO 2.2-m optical] and web-based database [2-micron All-Sky Survey] for all wavelengths. We have taken optical spectra {CTIO 1.5-m] and have a collaborator's radio data. By comparing the objects in NGC1788 to regions of known star formation, we can see how effective and what stage the star formation is in NGC1788. For example, we can compare color-color and color-magnitude diagrams (CCD and CMD, respectively) for our region (see Figures 4 and Figures 5 a & b) to those of known star formation. The majority of our objects fall above the main sequence on the CMD, implying that they are young.

In general, we are examining NGC1788 at multiple wavelengths so that we can assess the state of star formation in the region. The standard model of star formation in GMCs does not sufficiently explain the spatial distribution of stars in our universe. We endeavor to characterize effective star formation in an isolated cloud so that we might find a process to compliment the standard model.



References



1 Ramesh, B. MNRAS 276:923-932. 1995 Apr. 27. p293
2 Racine, Rene. AJ 71:233-45. 1968 May. p233
3 Ogura, Katsuo and Sugitani, Koji. Publ. Astron. Soc. Japan 15:91-98. 1998 Aug. p91





Figures and Tables



NGC1788: UBVRI false color image (13.5'x13.5') ~ I made this from data my advisor Stefanie and I took at CTIO’s 0.9-m telescope. We imaged NGC1788 in U, B, V, R, and I filters. Then I used Linux’s Gimp to color U, B, V, R and I images as purple, blue, green, red, and orange-red. I combined the short images with the long ones by having Gimp "multiply" the images’ pixel values; this enabled the point source details of the short exposures to combine somewhat with the nebulosity of the long ones. I changed the contrast of each filter image in order to bring out the most detail of the center region and the nebula. I then overlaid V, R, B, U, and I (in that order) on each other and combined them with the "screen" function, which created a nice blend. JPG (59 kb)

Figure 1: Molecular clouds of Orion complex (30° x 40°) ~ NGC1788/L1616 is cloud number 13 at about (5h 6m, -3d 30m). "Schematic diagram of the molecular clouds: the lowest contour from Fig. 2. Dots with numbers corresponding to those in Table 1, indicate locations of CO emission peaks. Some NGC numbers indicate the optical prominent objects coincident with CO peaks. The extent of UV emission from Banard’s loop is indicated by the shaded arc (from O’Dell, York, and Henize 1967; Isobe 1973). The dashed line roughly indicates the extent of the lamda Ori ring of clouds." (Maddalena, Ronald J., Morris, Mark, Moscowitz, J., and Thaddeus, P. The Large System of Molecular Clouds in Orion and Monoceros. ApJ 303:375-391. 1996 Apr. 1. p379) JPG (77 kb)

Figure 2a: Cometary clouds around Orion complex (15° x 25°) ~ NGC1788/L1616 is cloud number 3 at about (5h 5m, -3d 30m). The image shows how most nebulae seem to be being blown away from Sigma Orionis. Ramesh believes Epsilon Orionis is the star inducing the star formation. "Surface distribution of objects in Table 1. Ticks indicate the directions of their tails." (Ogura, Katsuo and Sugitani, Koji. "Remnant Molecular Clouds in the Ori OB 1 Association." Publ. Astron. Soc. 15:91-98. 1998 Aug. p97) JPG (49 kb)
Figure 2b: Cometary shape (DSS 1st Gen. 30'x30') ~ HD293815 is marked, as well as L1616 (the head) and L1615 (the tail). The image is a thirty by thirty arcminute Digital Sky Survey 1st generation image. JPG (114 kb)

Figure 3: Proper Motion (DSS 2nd Gen 15'x15') ~ Tentative proper motion vector image of objects around NGC1788 region. A Digital Sky Survey (DSS) image (North up and East left) of 15'x15' around NGC1788 tentatively marked with the proper motion vectors of the Hipparcos and Tycho Catalogues objects (HIC and TYC, respectively). The HIC objects have five digit number corresponding to the catalogue number while the TYC objects are arbitrarily numbered. The TYC has larger errors than the HIC, and as of now, this is just a temporary, rough visual of the motions of the objects. HIP23837 is a ROSAT X-ray source but from the distance eximate (658pc), probably is not part of the cluster (see Table 2). It is not known which are foreground, background and NGC1788 region objects. It could be that the cloud is disintegrating. JPG (75 kb)

Figure 4a: Near-Infrared Color-Color Diagrams ~ This for objects from the 2-micron All-Sky Survey (2MASS) Point Source Catalogue (PSC) in the central region (see Figure 5c). The x-ray sources (pink points) come from one of our collaborators, and we matched them visually on a 2MASS image, as well as coordinate wise to the PSC. The green and blue lines are the main sequence and giants III lines, respectively. Reddening vectors to account for dust obscuration have not been calculated. 2MASS has a faintness limit of 14.3 for K, which was not taken into account in these diagrams. JPG (29 kb)
Figure 4b: Color-Magnitude Diagram ~ This has the same properties as above (K is apparent magnitude). For the most part, the objects fall above the main sequence, as objects in a star-forming region should. The main sequence lines in this diagram were corrected for distance, which still needs to be verified. JPG (26 kb)
Figure 4c: CMD of IC348 ~ This is another star forming region, with main sequence line and reddening vectors (solid and dashed lines, respectively). (Lada, Elizabeth A. and Lada, Charles J. Near-Infrared Images of IC 348 and the Luminosity Functions of Young Embedded Star Clusters. AJ 109:1682-1996. 1995 Apr. p1687) JPG (58 kb)

Figure 5a: Optical Color-Color Diagrams ~ Color-color magnitude diagram. The optical data comes from one of our ESO collaborators. We calibrated their B, V, R, and I instrumental magnitudes with the transformation equations given and matched the objects in all filters. Once again, we took what we believe are the closest objects in ir.fits (see Figure 5.c). There are nine x-ray sources that have matches in the optical. The majority lie above the main sequence, as proto-stars or young stellar objects should. JPG (26 kb)
Figure 5b: Color-Magnitude Diagram ~ The tail end of the main sequence ends about M6. The X-ray sources are once again entering the main sequence. However, since this is optical, extinction is much worse than in the near-infrared. We have not taken that into account for this diagram. JPG (25 kb)
Figure 5c: Near-Infrared Image of Center ~ The objects in our CMD and CCD are taken from matching objects within the infrared image. The image is about 13’ long and 8’ wide. Only a few x-ray sources fall within this range, but they are all included in the diagrams. JPG (10 kb)
Figure 5d: Optical Image ~ This optical image is the same size, scale and location as the previous image. Comparing the two shows the extinction of the region. JPG (38 kb)

Figure 6a: Optical Spectra Part 1 ~ This is low-resolution optical spectra from CTIO's 1.5-m taken by Stefanie. They are of the ROSAT x-ray sources in our region of interest (see Table 1). We are looking for Lithium absorption at 6708Å, which is a characteristic of T-Tauri stars (young, low-mass stars). JPG (38 kb)
Figure 6b: Optical Spectra Part 2 ~ More x-ray optical spectra... JPG (38 kb)
Figure 6c: Optical Spectra Lithium ~ This is an enlargment of the 6700Å area to better see the Lithium absorption. JPG (38 kb)

Table 1: Multi-Wavelength Matches ~ View and/or download the table of correlated x-ray, near-infrared and optical objects. Compilation or DOC (177 kb)

Table 2: Proper Motions ~ View and/or download the table of Hipparcos and Tycho Catalogue sources in the central region of NGC1788 (such data as distance and proper motion vectors). The objects are also cross-listed with the X-Ray sources. Proper Motions or DOC (31 kb)

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