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Register a new userSearch for synchrotron emission from secondary leptons in dense cold starless cores
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D.I. Jones
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We report radio continuum observations with the Australia Telescope Compact Array of two molecular clouds. The impetus for these observations is a search for synchrotron radiation by cosmic ray secondary electrons/positrons in a region of enhanced density and possibly high magnetic field. We present modelling which shows that there should be an appreciable flux of synchrotron above the more diffuse, galactic synchrotron background. The starless core G333.125-0.562 and infrared source IRAS 15596-5301 were observed at 1384 and 2368 MHz. For G333.125-0.562, we find no significant levels of radio emission from this source at either frequency, nor any appreciable polarisation: we place an upper limit on the radio continuum flux from this source of 0.5 mJy per beam at both 1384 and 2368 MHz. Due to the higher than expected flux density limits, we also obtained archival ATCA data at 8640 MHz for this cloud and place an upper limit on the flux density of 50 micro-Jy per beam. Assuming the cosmic ray spectrum is similar to that near the Sun, and given the clouds molecular density and mass, we place an upper limit on the magnetic field of 500 micro-G. IRAS 15596-5301, with an RMS of 50 micro-Jy per beam at 1384 MHz, shows an HII region consistent with optically thin free-free emission already detected at 4800 MHz. We use the same prescription as G333 to constrain the magnetic field from this cloud to be less than 500 micro-G. We find that these values are not inconsistent with the view that magnetic field values scale with the average density of the molecular cloud.
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A point-like source of ~TeV gamma-rays has recently been seen towards the Galactic center by HESS and other air Cerenkov telescopes. In recent work (Ballantyne et al. 2007), we demonstrated that these gamma-rays can be attributed to high-energy protons that (i) are accelerated close to the event horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and (iii) finally interact to produce gamma-rays. The same hadronic collision processes will necessarily lead to the creation of electrons and positrons. Here we calculate the synchrotron emissivity of these secondary leptons in the same magnetic field configuration through which the initiating protons have been propagated in our model. We compare this emission with the observed ~GHz radio spectrum of the inner few pc region which we have assembled from archival data and new measurements we have made with the Australia Telescope Compact Array. We find that our model predicts secondary synchrotron emission with a steep slope consistent with the observations but with an overall normalization that is too large by a factor of ~ 2. If we further constrain our theoretical gamma-ray curve to obey the implicit EGRET upper limit on emission from this region we predict radio emission that is consistent with observations, i.e., the hadronic model of gamma ray emission can, simultaneously and without fine-tuning, also explain essentially all the diffuse radio emission detected from the inner few pc of the Galaxy.
We carry out an ALMA $rm N_2D^+$(3-2) and 1.3~mm continuum survey towards 32 high mass surface density regions in seven Infrared Dark Clouds with the aim of finding massive starless cores, which may be the initial conditions for the formation of massive stars. Cores showing strong $rm N_2D^+$(3-2) emission are expected to be highly deuterated and indicative of early, potentially pre-stellar stages of star formation. We also present maps of these regions in ancillary line tracers, including C$^{18}$O(2-1), DCN(3-2) and DCO$^+$(3-2). Over 100 $rm N_2D^+$ cores are identified with our newly developed core-finding algorithm based on connected structures in position-velocity space. The most massive core has $sim70:M_odot$ (potentially $sim170:M_odot$) and so may be representative of the initial conditions for massive star formation. The existence and dynamical properties of such cores constrain massive star formation theories. We measure the line widths and thus velocity dispersion of six of the cores with strongest $rm N_2D^+$(3-2) line emission, finding results that are generally consistent with virial equilibrium of pressure confined cores.
Understanding the role of magnetic fields in star-forming regions is of fundamental importance. In the near future, the exceptional sensitivity of SKA will offer a unique opportunity to evaluate the magnetic field strength in molecular clouds and cloud cores through synchrotron emission observations. The most recent Voyager 1 data, together with Galactic synchrotron emission and Alpha Magnetic Spectrometer data, constrain the flux of interstellar cosmic-ray electrons between $approx3$ MeV and $approx832$ GeV, in particular in the energy range relevant for synchrotron emission in molecular cloud cores at SKA frequencies. Synchrotron radiation is entirely due to primary cosmic-ray electrons, the relativistic flux of secondary leptons being completely negligible. We explore the capability of SKA in detecting synchrotron emission in two starless molecular cloud cores in the southern hemisphere, B68 and FeSt 1-457, and we find that it will be possible to reach signal-to-noise ratios of the order of $2-23$ at the lowest frequencies observable by SKA ($60-218$ MHz) with one hour of integration.
We report the identification of a sample of potential High-Mass Starless Cores (HMSCs). The cores were discovered by comparing images of the fields containing candidate High-Mass Protostellar Objects (HMPOs) at 1.2mm and mid-infrared (8.3um; MIR) wavelengths. While the HMPOs are detected at both wavelengths, several cores emitting at 1.2mm in the same fields show absorption or no emission at the MIR wavelength. We argue that the absorption is caused by cold dust. The estimated masses of a few 10^2Msun - 10^3 Msun and the lack of IR emission suggests that they may be massive cold cores in a pre-stellar phase, which could presumably form massive stars eventually. Ammonia (1,1) and (2,2) observations of the cores indicate smaller velocity dispersions and lower rotation temperatures compared to HMPOs and UCHII regions suggesting a quiescent pre-stellar stage. We propose that these newly discovered cores are good candidates for the HMSC stage in high-mass star-formation. This sample of cores will allow us to study the high-mass star and cluster formation processes at the earliest evolutionary stages.
(Abridged) We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by one or two orders of magnitude. At the same time, N2H+ has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and (2,2), and the 1.2 mm continuum. For each core we have built a model that fits simultaneously the radial profile of all observed emission and the central spectrum for the molecular lines. The observed abundance drops of CO and CS are naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to densities of several 10^5, while the NH3 abundance may be enhanced by reactions triggered by the disappearance of CO from the gas phase. With the help of our models, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence.
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