ترغب بنشر مسار تعليمي؟ اضغط هنا

Magnetic Field Structure of Dense Cores using Spectroscopic Methods

68   0   0.0 ( 0 )
 نشر من قبل Sayantan Auddy
 تاريخ النشر 2019
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

We develop a new core field structure (CFS) model to predict the magnetic field strength and magnetic field fluctuation profile of dense cores using gas kinematics. We use spatially resolved observations of the nonthermal velocity dispersion from the Green Bank Ammonia survey along with column density maps from SCUBA-2 to estimate the magnetic field strength across seven dense cores located in the L1688 region of Ophiuchus. The CFS model predicts the profile of the relative field fluctuation, which is related to the observable dispersion in direction of the polarization vectors. Within the context of our model we find that all the cores have a transcritical mass-to-flux ratio.



قيم البحث

اقرأ أيضاً

Theoretical and numerical works indicate that a strong magnetic field should suppress fragmentation in dense cores. However, this has never been tested observationally in a relatively large sample of fragmenting massive dense cores. Here we use the p olarization data obtained in the Submillimeter Array Legacy Survey of Zhang et al. to build a sample of 18 massive dense cores where both fragmentation and magnetic field properties are studied in a uniform way. We measured the fragmentation level, Nmm, within the field of view common to all regions, of 0.15 pc, with a mass sensitivity of about 0.5 Msun, and a spatial resolution of about 1000 au. In order to obtain the magnetic field strength using the Davis-Chandrasekhar-Fermi method, we estimated the dispersion of the polarization position angles, the velocity dispersion of the H13CO+(4-3) gas, and the density of each core, all averaged within 0.15 pc. A strong correlation is found between Nmm and the average density of the parental core, although with significant scatter. When large-scale systematic motions are separated from the velocity dispersion and only the small-scale (turbulent) contribution is taken into account, a tentative correlation is found between Nmm and the mass-to-flux ratio, as suggested by numerical and theoretical works.
The role played by magnetic field during star formation is an important topic in astrophysics. We investigate the correlation between the orientation of star-forming cores (as defined by the core major axes) and ambient magnetic field directions in 1 ) a 3D MHD simulation, 2) synthetic observations generated from the simulation at different viewing angles, and 3) observations of nearby molecular clouds. We find that the results on relative alignment between cores and background magnetic field in synthetic observations slightly disagree with those measured in fully 3D simulation data, which is partly because cores identified in projected 2D maps tend to coexist within filamentary structures, while 3D cores are generally more rounded. In addition, we examine the progression of magnetic field from pc- to core-scale in the simulation, which is consistent with the anisotropic core formation model that gas preferably flow along the magnetic field toward dense cores. When comparing the observed cores identified from the GBT Ammonia Survey (GAS) and Planck polarization-inferred magnetic field orientations, we find that the relative core-field alignment has a regional dependence among different clouds. More specifically, we find that dense cores in the Taurus molecular cloud tend to align perpendicular to the background magnetic field, while those in Perseus and Ophiuchus tend to have random (Perseus) or slightly parallel (Ophiuchus) orientations with respect to the field. We argue that this feature of relative core-field orientation could be used to probe the relative significance of the magnetic field within the cloud.
Magnetic and energetic properties are presented for 17 dense cores within a few hundred pc of the Sun. Their plane-of-sky field strengths are estimated from the dispersion of polarization directions, following Davis, Chandrasekhar and Fermi (DCF). Th eir ratio of mass to magnetic critical mass is 0.5-3, indicating nearly critical field strengths. The field strength B_pos is correlated with column density N as B_pos~N^p, where p=1.05+-0.08, and with density n as B_pos~n^q, where q=0.66+-0.05. These magnetic properties are consistent with those derived from Zeeman studies (Crutcher et al. 2010), with less scatter. Relations between virial mass M_V, magnetic critical mass M_B, and Alfven amplitude sigma_B/B match the observed range of M/M_B for cores observed to be nearly virial, with M/M_V=0.5-2, with moderate Alfven amplitudes, and with sigma_B/B=0.1-0.4. The B-N and B-n correlations in the DCF and Zeeman samples can be explained when such bound, Alfvenic, and nearly-critical cores have central concentration and spheroidal shape. For these properties, B~N because M/M_B is nearly constant compared to the range of N, and B~n^(2/3) because M^(1/3) is nearly constant compared to the range of n^(2/3). The observed core fields which follow B~n^(2/3) need not be much weaker than gravity, in contrast to core fields which follow B~n^(2/3) due to spherical contraction at constant mass (Mestel 1966). Instead, the nearly critical values of M/M_B suggest that the observed core fields are nearly as strong as possible, among values which allow gravitational contraction.
We compare the directions of molecular outflows of 62 low-mass Class 0 and I protostars in nearby (<450 pc) star-forming regions with the mean orientations of the magnetic fields on 0.05-0.5 pc scales in the dense cores/clumps where they are embedded . The magnetic field orientations were measured using the JCMT POL-2 data taken by the BISTRO-1 survey and from the archive. The outflow directions were observed with interferometers in the literature. The observed distribution of the angles between the outflows and the magnetic fields peaks between 15 and 35 degrees. After considering projection effects, our results could suggest that the outflows tend to be misaligned with the magnetic fields by 50+/-15 degrees in three-dimensional space and are less likely (but not ruled out) randomly oriented with respect to the magnetic fields. There is no correlation between the misalignment and the bolometric temperatures in our sample. In several sources, the small-scale (1000-3000 au) magnetic fields is more misaligned with the outflows than their large-scale magnetic fields, suggesting that the small-scale magnetic field has been twisted by the dynamics. In comparison with turbulent MHD simulations of core formation, our observational results are more consistent with models in which the energy densities in the magnetic field and the turbulence of the gas are comparable. Our results also suggest that the misalignment alone cannot sufficiently reduce the efficiency of magnetic braking to enable formation of the observed number of large Keplerian disks with sizes larger than 30-50 au.
We present a possible identification strategy for first hydrostatic core (FHSC) candidates and make predictions of ALMA dust continuum emission maps from these objects. We analyze the results given by the different bands and array configurations and identify which combinations of the two represent our best chance of solving the fragmentation issue in these objects. If the magnetic field is playing a role, the emission pattern will show evidence of a pseudo-disk and even of a magnetically driven outflow, which pure hydrodynamical calculations cannot reproduce.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا