Revealing H2D+ depletion and compact structure in starless and protostellar cores with ALMA

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the submillimeter dust continuum and H2D+ 1_{10}-1_{11} emission toward two evolved, potentially protostellar cores within the Ophiuchus molecular cloud, Oph A SM1 and SM1N. The data reveal small-scale condensations within both cores, with mass upper limits of M <~ 0.02M_Sun (~ 20 M_Jup). The SM1 condensation is consistent with a nearly-symmetric Gaussian source with a width of only 37 AU. The SM1N condensation is elongated, and extends 500 AU along its major axis. No evidence for substructure is seen in either source. A Jeans analysis indicates these sources are unlikely to fragment, suggesting that both will form single stars. H2D+ is only detected toward SM1N, offset from the continuum peak by ~150-200 AU. This offset may be due to either heating from an undetected, young, low luminosity protostellar source or first hydrostatic core, or HD (and consequently H2D+) depletion in the cold centre of the condensation. We propose that SM1 is protostellar, and that the condensation detected by ALMA is a warm (T ~ 30-50 K) accretion disk. The less concentrated emission of the SM1N condensation suggests that it is still starless, but we cannot rule out the presence of a low-luminosity source, perhaps surrounded by a pseudodisk. These data reveal observationally the earliest stages of the formation of circumstellar accretion regions, and agree with theoretical predictions that disk formation can occur very early in the star formation process, coeval with or just after the formation of a first hydrostatic core or protostar.

An analysis of the deuterium fractionation of star-forming cores in the Perseus molecular cloud

We have performed a pointed survey of N2D+ 2-1 and N2D+ 3-2 emission toward 64 N2H+-bright starless and protostellar cores in the Perseus molecular cloud using the Arizona Radio Observatory Submillimeter Telescope and Kitt Peak 12 m telescope. We find a mean deuterium fractionation in N2H+, R_D = N(N2D+)/N(N2H+), of 0.08, with a maximum R_D = 0.2. In detected sources, we find no significant difference in the deuterium fractionation between starless and protostellar cores, nor between cores in clustered or isolated environments. We compare the deuterium fraction in N2H+ with parameters linked to advanced core evolution. We only find significant correlations between the deuterium fraction and increased H_2 column density, as well as with increased central core density, for all cores. Towards protostellar sources, we additionally find a significant anti-correlation between R_D and bolometric temperature. We show that the Perseus cores are characterized by low CO depletion values relative to previous studies of star forming cores, similar to recent results in the Ophiuchus molecular cloud. We suggest that the low average CO depletion is the dominant mechanism that constrains the average deuterium fractionation in the Perseus cores to small values. While current equilibrium and dynamic chemical models are able to reproduce the range of deuterium fractionation values we find in Perseus, reproducing the scatter across the cores requires variation in parameters such as the ionization fraction or the ortho- to para-H_2 ratio across the cloud, or a range in core evolution timescales.

The Initial Conditions of Clustered Star Formation III. The Deuterium Fractionation of the Ophiuchus B2 Core

We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+ data reveal the deuterium fractionation in the high density gas across Oph B2. The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2, with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is consistent with previous results in isolated starless and protostellar cores. The column density distributions of both H2D+ and N2D+ show no correlation with total H2 column density. We find, however, an anticorrelation in deuterium fractionation with proximity to the embedded protostars in Oph B2 to distances >= 0.04 pc. Destruction mechanisms for deuterated molecules require gas temperatures greater than those previously determined through NH3 observations of Oph B2 to proceed. We present temperatures calculated for the dense core gas through the equating of non-thermal line widths for molecules (i.e., N2D+ and H2D+) expected to trace the same core regions, but the observed complex line structures in B2 preclude finding a reasonable result in many locations. This method may, however, work well in isolated cores with less complicated velocity structures. Finally, we use R_D and the H2D+ column density across Oph B2 to set a lower limit on the ionization fraction across the core, finding a mean x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using deuterated species as a probe of the physical conditions of dense gas in star-forming regions.

The Initial Conditions of Clustered Star Formation. II. N2H+ Observations of the Ophiuchus B Core

We present a Nobeyama 45 m Radio Telescope map and Australia Telescope Compact Array pointed observations of N2H+ 1-0 emission towards the clustered, low mass star forming Oph B Core within the Ophiuchus molecular cloud. We compare these data with previously published results of high resolution NH3 (1,1) and (2,2) observations in Oph B. We use 3D Clumpfind to identify emission features in the single-dish N2H+ map, and find that the N2H+ `clumps match well similar features previously identified in NH3 (1,1) emission, but are frequently offset to clumps identified at similar resolution in 850 micron continuum emission. Wide line widths in the Oph B2 sub-Core indicate non-thermal motions dominate the Core kinematics, and remain transonic at densities n ~ 3 x 10^5 cm^-3 with large scatter and no trend with N(H2). Non-thermal motions in Oph B1 and B3 are subsonic with little variation, but also show no trend with H2 column density. Over all Oph B, non-thermal N2H+ line widths are substantially narrower than those traced by NH3, making it unlikely NH3 and N2H+ trace the same material, but the v_LSR of both species agree well. We find evidence for accretion in Oph B1 from the surrounding ambient gas. The NH3/N2H+ abundance ratio is larger towards starless Oph B1 than towards protostellar Oph B2, similar to recent observational results in other star-forming regions. Small-scale structure is found in the ATCA N2H+ 1-0 emission, where emission peaks are again offset from continuum emission. In particular, the ~1 M_Sun B2-MM8 clump is associated with a N2H+ emission minimum and surrounded by a broken ring-like N2H+ emission structure, suggestive of N2H+ depletion. We find a strong general trend of decreasing N2H+ abundance with increasing N(H2) in Oph B which matches that found for NH3.

The Initial Conditions of Clustered Star Formation I: NH3 Observations of Dense Cores in Ophiuchus

We present combined interferometer and single dish telescope data of NH3 (J,K) = (1,1) and (2,2) emission towards the clustered star forming Ophiuchus B, C and F Cores at high spatial resolution (~1200 AU) using the Australia Telescope Compact Array, the Very Large Array, and the Green Bank Telescope. While the large scale features of the NH3 (1,1) integrated intensity appear similar to 850 micron continuum emission maps of the Cores, on 15 (1800 AU) scales we find significant discrepancies between the dense gas tracers in Oph B, but good correspondence in Oph C and F. Using the Clumpfind structure identifying algorithm, we identify 15 NH3 clumps in Oph B, and 3 each in Oph C and F. Only five of the Oph B NH3 clumps are coincident within 30 (3600 AU) of a submillimeter clump. We find v_LSR varies little across any of the Cores, and additionally varies by only ~1.5 km/s between them. The observed NH3 line widths within the Oph B and F Cores are generally large and often mildly supersonic, while Oph C is characterized by narrow line widths which decrease to nearly thermal values. We find several regions of localized narrow line emission (Delta v < 0.4 km/s), some of which are associated with NH3 clumps. We derive the kinetic temperatures of the gas, and find they are remarkably constant across Oph B and F, with a warmer mean value (T_K = 15 K) than typically found in isolated regions and consistent with previous results in clustered regions. Oph C, however, has a mean T_K = 12 K, decreasing to a minimum T_K = 9.4 K towards the submillimeter continuum peak, similar to previous studies of isolated starless cores. There is no significant difference in temperature towards protostars embedded in the Cores. [Abridged]