Are Prestellar Cores Magnetically-Subcritical?
NAOJ, Mitaka, JP
Magnetic fields play an important role in the process of star formation.
However, it remains uncertain how important the magnetic field is.
Previous Zeeman observations are mainly done with HI and OH. However,
HI and OH do not trace dense gas but only trace the less-dense gas in clouds
or envelopes of dense cores.
CN traces the dense gas with 105 cm-3.
But, CN Zeeman observations are
limited toward high-mass-star-forming regions where already stars are formed.
To constrain the role of magnetic field in the process of gravitational contraction of cores, we carried out the Zeeman observations using the Nobeyama 45-m telescope. Our target line is a CCS (JN=43-32) line at 45 GHz. CCS is abundant only in the PRESTELLAR phase and the critical density is about 104 cm-3. CCS also show relatively large Zeeman splitting. Therefore, CCS is the best line to trace the magnetic field associated with dense cores. To do the Zeeman observations, we first developed the 45 GHz-band dual-linear polarization receiver and a spectrometer with 60 Hz resolution, and installed them in the Nobeyama 45-m system.
Then we conducted the CCS (43-32) Zeeman observations toward TMC-1, a well-studied prestellar core in Taurus molecular cloud. We simultaneously observed CCS (JN=43-32) and HC3N (J=5-4) and detected the Stokes V spectrum of CCS with 135 GHz splitting of s/n ~ 9, and no split for HC3N spectrum. The HC3N line is a non-Zeeman line, and no-detection of HC3N Stokes-V verifies the robust detection of CCS Zeeman splitting. The detected split 135 GHz of CCS corresponds to the field strength of 211 micro Gauss. The density of gas traced with CCS is estimated to be 3 x 104 cm-3. from the LVG analysis using multi-line data. The estimated plasma beta is 0.01. Using this value, the flux-to-mass ratio is estimated to be more than twice the critical value, where we used the column density of a few x 1022 cm-3 estimated from C18O and extinction data. If we take into accout of the inclination assuming a plan-parallel sheet-like geometry, the flux-to-mass ratio is 5. In other words, the TMC-1 is magnetically subcritical. This is the first discovery of a magnetically subcritical core.
By May, we plan to observe one or two more prestellar cores. So, I will include results of these additional observations in my presentation. In my presentation, I will discuss the importance of magnetic field in the evolution of prestellar cores, based on our observational results.
S. Kameno, JAO/NAOJ, JP
K. Dobashi, Tokyo Gakugei U, JP
T. Shimoikura, Tokyo Gakugei U, JP
R. Kawabe, NAOJ, JP
S. Yamamoto, U Tokyo, JP
Suggested Session: Magnetic Fields