ALMA Observations of Low-Mass End of A Core Mass Function in Rho Ophiuchus A Region Fumitaka Nakamura (National Astronomical Observatory of Japan) collaborators Chihomi Hara (Univ. of Tokyo), Takeshi Kamazaki (NAOJ), Ryohei Kawabe (NAOJ), Motohide Tamura (Univ. of Tokyo), Shigehisa Takakuwa (Kagoshima Univ./ASIAA), Naomi Hirano (ASIAA), Rachel Freisen (Univ. of Toronto), James Di Francesco (NRC)

Outline of My Talk Introduction What is the basic unit of star formation? ALMA Observations cycle-2 Latest Results Conclusions

What is the Basic Unit of Star Formation? Stars form from dense cores in molecular clouds. The dense cores typically have densities of 104-105 cm-3 and sizes of 0.1 pc. Dense core: Barnard 68 If stars form from dense cores, the mass functions of dense cores should be similar to the stellar IMF.

Core Mass Function The observed core mass functions resemble the stellar IMF. Motte et al. (1998) ~10” resolution Pipe Nebula: Alves et al. (2007) These observations suggest that a dense core is the basic unit of star formation.

Substructure in Dense Cores (SMA) 1.1-mm continuum AzTEC/ASTE + SMA 850um image 850 um + SMA ~3” resolution Nakamura et al. (2012) Dense cores identified by Motte et al. (1998) may not be a basic unit of star formation. (see, however, Schnee wt al (2010, 2012) and Duham et al. (2016)

Our Target: Rho Ophiuchus cluster-forming clump Most stars are formed in clustered environments. Nearest Cluster-Forming Region ~120 pc Many outflows are detected  active cluster-forming region Many protostars and BD candidates are found from infrared observtions (e.g., Spitzer, SUBARU). CO (1-0) integrated intensity map (Nakamura et al. 2011)

ALMA Observations Cycle-2 Band-6 Observations (7-m+12-m) Targets: 1.3-mm continuum 12CO(2-1), 13CO(2-1), C18O(2-1) Angular resolution achieved 1-2 arcsec ~ 120- 240 AU @ 120 pc Mosaic observations towards three brightest regions A 150 12-m pointings B2 149 12-m pointings C 138 12-m pointings Papers preparation Hara et al. (2016, in prep.) Oph A protostars Kawabe et al. (2016, in prep.) Oph A protostars Kamazaki et al. (2016, in prep.) Oph B2 Nakamura et al. (2016, in prep.) prestellar cores in A, B2, C

Continuum image of Oph A Angular resolution ~ 1.”2 x 0.7 ” Rms ~ 0.25 mJy/beam Contour: CO (2-1) 7m+12m VLA1623 Compact continnum sources (a new protostar and proto-BD) Starless structures 500 AU Contours = 850mm

Blow-up of the Oph A Ridge SM1, SM2, A-MM1, … are the cores identified by Motte et al. (1998) Contour = 4s, 8s, 12s,… (blue~10s, green~30s, red > 50s) 1s ~ 0.25 mJy/beam A-MM6 A-MM1 1000 AU~0.0045 pc SM1N SM1 A-MM2 VLA1623 (Class 0) A-MM3 SM2 beam size =1.2” x 0.7” 1.3-mm continuum (7-m +12-m)

Core analysis - Clumpfind After trimming the edges where noises tend to be large, we identify ~60 small structures, by applying clumpfind. Here we tentatively call the structures identified as starless cores. Clumpfind Clumpfind parameters: Threshold = 5s Delta = 5s 1s = 0.25 mJy/beam Beam=1.2” x 0.7” Williams, de Geus & Leo Blitz (1994) Many small starless cores are found.

Core Radius Histogram of radii of cores identified with clumpfind (structures > 5s) 15 10 5 Radius (pc) Core radius ranges from 500 AU (0.0025 pc) to 3000AU (0.01 pc)

Core Mass and Density Mean mass ~ 0.1 M8 Density ~ 2 x 109 cm-3 20 20 15 15 10 10 5 5 Density (cm-3) Mass (M8) Mean mass ~ 0.1 M8 Density ~ 2 x 109 cm-3 Assuming gas-to-dust ratio =100 Beta = 2

Are Cores Gravitationally-Bound? Jeans mass is the minimum mass below which thermal pressure can support against gravity. Jeans mass at 109cm-3 = 0.06 M8 Jeans length = 116 AU (~ 0.9”) Temperature ~ 18 K. (Di Francesco et al. 2004)

Core Mass Function? CMF appears to follow the same power law as Salpeter IMF down to 0.1M8. The turn over mass is about 0.1M8.

Summary (1) I presented preliminary results of our ALMA cycle-2 mosaic observations toward rho Ophiuchus A region, the nearest cluster- forming region. (2) Applying clumpfind, we identified about 60 condensations. (3) Their typical size, mass, density are 500 AU, 0.1 M8 and 109 cm-3, respectively. (4) We uncover the low-mass end of the core mass function in this region. The turn over mass is 0.1 M8. Hara et al. (in prep.), Kamazaki et al. (in prep), Kawabe et al. (in prep.), Nakamura et al. (in prep), …..

ALMA cycle-1 Observations of Prestellar Cores in 3mm No 3-mm detection FHSC candidate Chamaeleon-MMS1 Extended 3mm continuum detected Protostars 19 cores with 3mm continuum (point-like) 25 total protostellar candidates detected new YSO candidates 7 not found in Dunham et al. (2013) 2 not found in Winston et al. (2012) Angular resolution = 3” Prestellar core Protostellar core (Dunham et al. 2016) (see also Schnee et al. 2010, 2012)