This section tells the reader all about how you conducted the research to get the new data you are presenting. For purely theoretical papers there is no 'Observation' section, but usually a section that presents the basic outline of the theoretical modeling and the basic equations that comprise such a research program.
In the text below, I elected to summarize what was known about each of the objects in my study, plus the new information I had culled from sources such as the Palomar Observatory Sky Survey and the infrared observations I had extracted from the database of the Infrared Astronomical Satellite (IRAS).
The region of the sky associated with each cometary cloud was examined using photographs from the Palomar Observatory Sky Survey and from the IIIaJ, SRC Southern Extension. The optical appearances of the 15 cometary clouds readily fall into one of three categories:
Category I clouds not only show a pronounced cometary appearance outlined by bright, diffuse nebulosity, but the region identified as the nucleus contains one or more embedded stars or star forming regions. The members of this group are: G96-15, G110-13, G315+21, G354+24, G359-17 and are displayed in Fig. 1.
Category II clouds appear as faint, diffuse, reflection nebulae of nearly constant optical surface brightness, but show a clumpy internal structure. This group, shown in Fig. 2, includes: G64-26, G208-28, G225-66, G228-27, G239-15 and the Draco cloud, G90+38.
For Category III clouds, there are no discernable signs of the far-IR cloud either as a diffuse, low surface brightness nebulosity, or as a region of depressed star density characteristic of obscurring clouds. The clouds in this category are: G139-65, G192-67, G206+24, G213+26 and are presented in Fig. 3.
a) Category I:
G96-15
The prominent nucleus of this object is associated with the A7e emission-line star LkH-alpha-233 (Allen, 1973; Herbig and Rao, 1972). The far-IR image shows that LkH-alpha-233 is connected to a diffuse, isolated cloud near RA = 22h 15m and Dec = +44d by a narrow filament of emission containing several clumps. There is a tendency for the clumps to become more diffuse as the filament is followed from LkH-alpha-233 to the northern cloud. An object similar to this is the cometary cloud CG4 described by Reipurth(1983). As for the Draco cloud, G96-15 has the appearance of a cloud that is breaking up or shedding mass.
An IR study by Cohen(1974) showed excess IR emission from LkH-alpha-233 indicating the presence of circumstellar dust. It was also identified by Cohen as a stellar object associated with a 'biconical or hour glass nebula '. Assuming that the embedded mV = +13.8 star has a luminosity class V with MV = +2.3 and B-V = +0.88 ( Strom et al, 1973), then AV = +1.8, and d = 860 pc. If the optical extinction derived from the far-IR column density in Table 2 is used, AV = +5.8, and d = 140 pc. The range of distances is, therefore, 140 - 860 pc. The lower value is adopted in order to obtain lower limits to the cloud parameters.
G110-13
An IR study by Cohen(1974) showed excess The optical appearance of this cloud is strikingly similar to the globule CG 15 described by Reipurth(1983). G110-13 contains the embedded B9 star SAO 53209 with mV = +9.3. Assuming that it is a main sequence star with MV = +0.3, and that the extinction is as large as suggested by the far-IR column density (Table 2) of AV = +2.4, then d = 200 pc. As a supergiant of luminosity class I, one obtains a distance between 3.8 and 11 kilo parsecs corresponding to an elevation above the Galactic plane of between 850 and 2500 parsecs. This is substantially higher than expected for B-type Pop I stars. We therefore adopt 200 pc as the best available distance estimate.
G315+21
This object is coincident with the bright emission nebula NGC- 5367 mapped by van Till, Loren and Davis(1975; hereafter TLD) in several isotopic CO lines of the J=1 - 0 transition. They obtained VLSR = -6.0 + / - 0.3 km/s and D V = 2.0 + / - 1.6 km/s along with a peak value for T* = 20.0 + / - 1.6 K at RA = 13h 54.7m and Dec = -39d 43.7'. The distance to this cloud was estimated as 200 - 400 pc depending on whether the cloud is assumed to be within the 200 pc HI layer in the solar vicinity, or whether its VLSR may be reliably used to obtain a kinematic distance ( 400 pc). For a mean distance of 300 pc, TLD also derive a total mass of M = 30 M(sun) , a size of 1 pc, and nH2 = 1000 cc. G315+21 is also found in the survey of molecular gas at high galactic latitudes by MMB and MBM and is listed in the latter work as cloud No. 112.
There are two stars near NGC-5367 possibly associated with this nebula which have been tentatively identified as mid-to-late B-type stars by TLD. The brighter of the two has mV = +9.53 and is located at the density maximum of the cloud where the gas kinetic temperature is also the highest. NGC-5367 appears to be a relatively low-mass, high latitude molecular cloud undergoing recent star formation wherein at least one B-type star may have recently formed.
G354+24
G354+24 can be identified in POSS-E photographs as the prominent optical emission nebulae IC 4592 and IC 4601. Projected against IC 4592 is the mv = +4.0, B2 IV - V star, iota Sco. According to the Yale Bright Star Catalog, B-V = +0.04, however for a star of this spectral type, (B-V) = -0.24 (Johnson, 1966) so E(B-V) = 0.28 and AV = 0.84. The extinction-corrected distance is then 135 pc, similar to the value cited by Yeroshevich(1971). A second star embedded in the nebulosity of IC 4601 is SAO 184309, an mV = +8.08, B9.5V star. Assuming MV = +0.5 and AV = 2.0 one also obtains d = 130 pc. The optical polarization study by Yeroshevich(1971) shows that the plane of the polarization is parallel to the edge of the optical emission extending NW-SE across the nebula suggesting the presence of a large- scale magnetic field with the same position angle.
G359-17
G359-17 is the well known R CrA association. R CrA is an emission-line A-type star located near the apex of the cometary reflection nebula NGC 6729 at RA = 18h 58m 32.5s and Dec = -37d 2.1' with the young F0e star, T CrA, located near the southern edge of the nebula. NGC 6726/7 northwest of R CrA, also includes the young B- type, emission-line variable TY CrA. The R CrA association is located at a distance of 150 pc ( Gaposchkin and Greenstein, 1936). Millimeter-wave observations by Loren(1979) show that the dominant heating source for this region is near TY CrA. Since no velocity gradient along the major axis of the cloud has been detected, rotation appears to be ruled-out as the reason for its flattened appearance. Optical polarization studies by Vrba(1976, 1977) suggest that elongation of the cloud may be due to the streaming of material along a magnetic field with an implied strength in the nuclear region of 150 micro Gauss ( Vrba, Coyne and Tapia, 1981). The orientation of the field is parallel to the optical filaments and to the northern edge of the cloud. The polarization and optical extinction data are also consistent with the presence of dust grains that are much larger than those found in the ISM. The mass of the ~0.5 x 0.7 pc nucleus was estimated by Loren, Sandqvist and Wootten(1983) as ~ 130 M(sun). Far-IR studies by Wilking et al(1985) have identified ~50 embedded stars, implying a star formation efficiency in the nuclear region between 14 and 56 percent depending on the masses assumed for the embedded stars.
b) Category II:
G64-26
This cloud has a V-shaped appearance at 100 microns with a vertex near RA = 21h 26.7m, Dec = +11.5d with filaments extending northward. POSS photographs show no prominent optical features associated with the western filament, though the broadest portion of the eastern one is coincident with the globular cluster M-15 and is adjacent to the bright star SAO 107195. This star has a weak reflection nebula extending northwards, including a secondary embedded object near RA = 21h 29.2m and Dec = +21d. There are several point-like objects at 100 microns within both filaments and near M-15.
G208-28
Although its 'tail' component is coincident with prominent optical nebulosity, the nuclear region near RA = 5h 00m and Dec = -9d shows no such emission. The secondary nuclear peak at RA = 4h 58.3m and Dec = -9.3d, identified as DG 049, coincides with an embedded, star-like object. The ribbon-like, twisted tail connects the nuclear regions to a large diffuse object near RA = 4h 33.5m, Dec = -14d 44.3'. The CO survey by MBM was able to detect the two nuclei whose VLSR = 4.2 and 4.8 km/s as Cloud #22. The diffuse cloud feature was observed as Cloud #20 at VLSR = 0.3 km/s.
G225-66
This striking object appears in the IIIaJ Schmidt photograph SRC-416-J and is near the bright star iota2 Fornacis. It has a low surface brightness in the J-band photograph with only the 'head' being clearly visible. At 100 microns, G225-66 can be resolved into two nuclei, each having its own sinuous tail extending about 14 degrees to the south, terminating in a diffuse clump of 100 micron emission.
G228-27
G228-27 is another V-shaped cloud with two bright nuclei connected to a common diffuse feature at the vertex. The southern nucleus also shows a connection with a second diffuse clump. Both of the nuclei coincide with optical emission. The star SAO 170450 appears to be embedded in the northern nucleus, though its true physical relationship with that feature remains unclear.
G239-15
At 100 microns, G239-15 is morphologically similar to G208-28 and G354+24, however, it does not coincide with prominent optical emission. The 6 SAO stars encompassed by the lowest 100 micron brightness contour, SAO 196964, 196967, 196981, 197016 and 197020, show no evidence for reflection nebulosity and may be unrelated field stars.
c) Category III:
G139-65
This V-shaped object has two tails extending about 2 degrees south of a vertex located near RA = 1h 15m and Dec = -4d. The eastern tail is resolved at 100 microns into 4-5 knots reminescent of vortices shed by objects moving through a fluid at intermediate Reynolds number. There are no prominent optical features in this field corresponding to the cloud, nor are their bright stars with reflection nebulae.
G192-67
Although its major morphologic elements at 100 microns are amorphous, the feature at RA = 2h 18m and Dec = -17d gives this cloud a head-tail appearance. The POSS print shows no indication of diffuse optical emission near this cloud.
G206+24
At 100 microns, this V-shaped cloud situated near ... Cancri extends 2.5 degrees along each arm. The vertex near RA = 8h 8.3m and Dec = 15d 50' contains two faint stars, neither showing evidence for nebulosity. The optical field shows no trace of the 100 micron cloud.
G213+26
G213+26 is a dumb bell-shaped cloud and has a very striking appearance with its two prominent lobes apparently connected by 3 narrow filaments. The larger of the the two components at RA = 8h 30m and Dec = +10d has a rounded shape reminiscent of supernovae remnants. There is no evidence for optical emission from this cloud on POSS photographs. There are three stars coincident with the southeastern lobe, SAO 6849, 116863 and 97902. However, it is not obvious from their photographic images that they are related to the cloud.
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