E. F. Ladd

We calculated bolometric temperature (T<SUB>bol</SUB>) and luminosity (L<SUB>bol</SUB>) for 128 young stellar objects (YSOs) in Taurus, 74 in the Ophiuchus 'core', and 33 in the Ophiuchus 'off-core' region. We have constructed the bolometric luminosity-temperature (BLT) diagram, the log-log plot of L<SUB>bol</SUB> versus T<SUB>bol</SUB>, for the three samples. T<SUB>bol</SUB> is defined as the temperature of a blackbody having the same frequency as the observed continuum spectrum. It measures the redness (or coldness) of an astronomical source. The BLT diagram is analogous to the H-R diagram and allows for a direct and quantitative comparison of YSOs at a wide variety of evolutionary states, ranging from the most deeply embedded stars to T Tauri stars nearly on the main sequence. We found (1) T<SUB>bol</SUB> increases monotonically from embedded sources (approximately 60-500 K) to classical T Tauri stars (approximately 1000-3000 K) to weak-line T Tauri stars (approximately 2000-5000 K); (2) T<SUB>bol</SUB> correlates reasonably well with the age inferred from the evolutionary models of pre-main-sequence stars and protostars for embedded 'protostars' and weak-line T Tauri stars. There is no significant correlation for the classical T Tauri stars. These results can be understood in terms of dissipation of circumstellar dust envelope and disk during the early stages of stellar evolution. Sources in the three regions have different distributions in the BLT diagram. The Ophiuchus core has the highest fraction of cold sources among the three regions. These cold sources are also more luminous than the YSOs in the other regions. The Ophiuchus off-core sample is dominated by the more evolved pre-main-sequence stars. The Taurus sources have distributions intermediate in L<SUB>bol</SUB>, T<SUB>bol</SUB>, and age between the Ophiuchus core and off-core distributions. These may suggest differences in the star formation history, and possibly in the stellar masses and mass accretion rates in these star-forming regions.

We propose the 'bolometric temperature' T(bol) as a measure of the circumstellar obscuration and evolutionary development of a young stellar object (YSO). T(bol) is the temperature of a blackbody having the same mean frequency as the observed continuum spectrum. We present three indications that a YSO evolves toward the main sequence from low to high T(bol) as a YSO clears its natal circumstellar dust: (1) For 129 YSOs in Taurus-Auriga, T(bol) ranges continuously from 60 to 5250 K, from 'protostars' to 'classical' T Tauri stars (CTTs) to 'weak-line' T Tauri stars (WTTs), and a plot of L(bol) versus T(bol) terminates abruptly at the main sequence. (2) In photospheric temperature T(eff) CTTs and WTTs are indistinguishable, with T(eff) about 4200 K, but in T(bol) WTTs are distinctly hotter (3600 K) than CTTs (2100 K). These temperatures indicate that circumstellar matter intercepts a larger fraction of the stellar luminosity for CTTs (0.5) than for WTTs (0.2). (3) In stellar groups, YSOs with low T(bol) are fewer and more concentrated, while YSOs with high T(bol) are more numerous and widespread. As T(bol) increases, an increasing fraction of YSOs lie outside a fiducial contour of (C-13)O line emission. Thus colder YSOs are probably younger and hotter YSOs older than the dispersal time for gas traced by the (C-13)O line, estimated to be 1-3 Myr.

We present a complete survey of current star formation in the Perseus molecular cloud, made at 850 and 450 with SCUBA at the JCMT. Covering 3 deg2, this submillimetre continuum survey for protostellar activity is second in size only to that of ρ Ophiuchus (Johnstone et al. 2004, ApJ, 611, L45). Complete above 0.4 ( detection in a beam), we detect a total of 91 protostars and prestellar cores. Of these, 80% lie in clusters, representative of star formation across the Galaxy. Two of the groups of cores are associated with the young stellar clusters IC 348 and NGC 1333, and are consistent with a steady or reduced star formation rate in the last 0.5 Myr, but not an increasing one. In Perseus, 40–60% of cores are in small clusters (<50 ) and isolated objects, much more than the 10% suggested from infrared studies. Complementing the dust continuum, we present a C18O map of the whole cloud at resolution. The gas and dust show filamentary structure of the dense gas on large and small scales, with the high column density filaments breaking up into clusters of cores. The filament mass per unit length is 5–11 per 0.1 pc. Given these filament masses, there is no requirement for substantial large scale flows along or onto the filaments in order to gather sufficient material for star formation. We find that the probability of finding a submillimetre core is a strongly increasing function of column density, as measured by C18O integrated intensity, . This power law relation holds down to low column density, suggesting that there is no Av threshold for star formation in Perseus, unless all the low- submm cores can be demonstrated to be older protostars which have begun to lose their natal molecular cloud.

Images in the 1-0 S(1) line of molecular hydrogen at 2.12 microns of six extremely young stellar objects are presented: NGC 1333 IRAS 2 and 4, L1551 IRS 5 and NE, L1634, and IRAS 08076-3556. In all cases, we find H<SUB>2</SUB> 1-0 S(1) line emission from bipolar jets of various degrees of collimation. In two sources (L1634 and IRAS 08076-3556), morphological arguments strongly suggest that previously made identifications of the outflow source with near-infrared sources have to be revised. In L1634 we find evidence for a second, previously undetected outflow. IRAS 08076-3556 may also have a second outflow, although our data are less conclusive. The outflow in L1551 NE and the double outflow associated with NGC 1333 IRAS 2 are confirmed by our imaging data. Multiple outflow activity is seen toward all of the regions imaged here. We speculate that the formation of small groups of stars within one outflow lifetime is a rather common occurrence.

Images of <SUP>12</SUP>CO and <SUP>13</SUP>CO J = 1-0 emission from the molecular clouds associated with the optical H II regions Sh 155, Sh 235, and Sh 140 are presented to better understand the interstellar gas conditions associated with regions of massive star formation. In the vicinity of the H II regions, there is evidence for the compression of ambient molecular material from shocks associated with the expansion of the ionized gas component and the near complete photoionization or photodissociation of molecular material within the H II region. However, the images also show that most of the molecular mass of the clouds resides within the extended, low column density regions well removed from the localized sites of star formation. Each cloud is characterized by a global line width which is dominated by the relative motions between resolved cloud substructures rather than motions along the line of sight inferred from individual profile line widths. We identify a relationship of the observed profile line widths with molecular gas column density which is consistent with opacity broadening of an intrinsic line width nearly independent with column density. Thus any tendency for neighboring emitting components to be at the same velocity (i.e., spatial coherence to the velocity field) must be weak or absent within CO-emitting regions of these clouds. In the context of a clumpy cloud medium, we use the variations of profile line width and antenna temperature with column density to derive the following limits for clump properties: clump masses less than 0.01 M<SUB>sun</SUB>, clump sizes less than 4 x 10<SUP>16</SUP> cm, and clump volume densities greater than 1 x 1O<SUP>4</SUP> cm<SUP>-3</SUP>.