1. 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)

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

3. What is the Basic Unit of Star Formation?
Stars form from dense cores in molecular clouds.
The dense cores typically have densities of 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.

4. 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.

5. 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)

6. 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)

7. 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 ~ pc
Mosaic observations towards three brightest regions
A m pointings
B m pointings
C 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

8. 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

9. 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~ 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)

10. 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.

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

12. Core Mass and Density Mean mass ~ 0.1 M8 Density ~ 2 x 109 cm-3 20 2015 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

13. 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)

14. 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.

15. 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), …..

16. 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)