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Register a new userSKA studies of in-situ synchrotron radiation from molecular clouds
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Observations of the properties of dense molecular clouds are critical in understanding the process of star-formation. One of the most important, but least understood, is the role of the magnetic fields. We discuss the possibility of using high-resolution, high-sensitivity radio observations with the SKA to measure for the first time the in-situ synchrotron radiation from these molecular clouds. If the cosmic-ray (CR) particles penetrate clouds as expected, then we can measure the B-field strength directly using radio data. So far, this signature has never been detected from the collapsing clouds themselves and would be a unique probe of the magnetic field. Dense cores are typically ~0.05 pc in size, corresponding to ~arcsec at ~kpc distances, and flux density estimates are ~mJy at 1 GHz. The SKA should be able to readily detect directly, for the first time, along lines-of-sight that are not contaminated by thermal emission or complex foreground/background synchrotron emission. Polarised synchrotron may also be detectable providing additional information about the regular/turbulent fields.
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Previous studies have shown that star formation depends on the driving of molecular cloud turbulence, and differences in the driving can produce an order of magnitude difference in the star formation rate. The turbulent driving is characterised by the parameter $zeta$, with $zeta=0$ for compressive, curl-free driving (e.g. accretion or supernova explosions), and $zeta=1$ for solenoidal, divergence-free driving (e.g. Galactic shear). Here we develop a new method to measure $zeta$ from observations of synchrotron emission from molecular clouds. We calculate statistics of mock synchrotron intensity images produced from magnetohydrodynamic simulations of molecular clouds, in which the driving was controlled to produce different values of $zeta$. We find that the mean and standard deviation of the log-normalised synchrotron intensity are sensitive to $zeta$, for values of $zeta$ between $0$ (curl-free driving) and $0.5$ (naturally-mixed driving). We quantify the dependence of zeta on the direction of the magnetic field relative to the line of sight. We provide best-fit formulae for $zeta$ in terms of the log-normalised mean and standard deviation of synchrotron intensity, with which $zeta$ can be determined for molecular clouds that have similar Alfvenic Mach number to our simulations. These formulae are independent of the sonic Mach number. Signal-to-noise ratios larger than $5$, and angular resolutions smaller than $5%$ of the cloud diameter, are required to apply these formulae. Although there are no firm detections of synchrotron emission from molecular clouds, by combining Green Bank Telescope and Very Large Array observations it should be possible to detect synchrotron emission from molecular clouds, thereby constraining the value of $zeta$.
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.
Using a suite of radiation hydrodynamic simulations of star cluster formation in turbulent clouds, we study the escape fraction of ionizing (Lyman continuum) and non-ionizing (FUV) radiation for a wide range of cloud masses and sizes. The escape fraction increases as H II regions evolve and reaches unity within a few dynamical times. The cumulative escape fraction before the onset of the first supernova explosion is in the range 0.05-0.58; this is lower for higher initial cloud surface density, and higher for less massive and more compact clouds due to rapid destruction. Once H II regions break out of their local environment, both ionizing and non-ionizing photons escape from clouds through fully ionized, low-density sightlines. Consequently, dust becomes the dominant absorber of ionizing radiation at late times and the escape fraction of non-ionizing radiation is only slightly larger than that of ionizing radiation. The escape fraction is determined primarily by the mean $langle taurangle$ and width $sigma$ of the optical-depth distribution in the large-scale cloud, increasing for smaller $langle taurangle$ and/or larger $sigma$. The escape fraction exceeds (sometimes by three orders of magnitude) the naive estimate $e^{-langle taurangle}$ due to non-zero $sigma$ induced by turbulence. We present two simple methods to estimate, within $sim20%$, the escape fraction of non-ionizing radiation using the observed dust optical depth in clouds projected on the plane of sky. We discuss implications of our results for observations, including inference of star formation rates in individual molecular clouds, and accounting for diffuse ionized gas on galactic scales.
Nuclear resonant forward scattering (NFS) and nuclear inelastic scattering (NIS) of synchrotron radiation are fairly recent spectroscopic methods for the investigation of complexes containing Mossbauer-active transition metal ions. NFS, which can be regarded as Mossbauer spectroscopy in the time domain, overcomes some limitations of conventional Mossbauer spectroscopy as has been demonstrated especially for bioinorganic compounds. NIS extends the energy range of conventional Mossbauer spectroscopy to the range of molecular vibrations. Since NIS is sensitive only to the mean-square displacement of Mossbauer nuclei it can be used as site-selective vibrational spectroscopy. It complements usefully comparable techniques such as IR or Raman spectroscopy. Examples are given for applications to spin crossover complexes, nitroprusside compounds, heme model complexes and myoglobin.
UV radiation feedback from young massive stars plays a key role in the evolution of giant molecular clouds (GMCs) by photoevaporating and ejecting the surrounding gas. We conduct a suite of radiation hydrodynamic simulations of star cluster formation in marginally-bound, turbulent GMCs, focusing on the effects of photoionization and radiation pressure on regulating the net star formation efficiency (SFE) and cloud lifetime. We find that the net SFE depends primarily on the initial gas surface density, $Sigma_0$, such that the SFE increases from 4% to 51% as $Sigma_0$ increases from $13,M_{odot},{rm pc}^{-2}$ to $1300,M_{odot},{rm pc}^{-2}$. Cloud destruction occurs within $2$-$10,{rm Myr}$ after the onset of radiation feedback, or within $0.6$-$4.1$ freefall times (increasing with $Sigma_0$). Photoevaporation dominates the mass loss in massive, low surface-density clouds, but because most photons are absorbed in an ionization-bounded Str{o}mgren volume the photoevaporated gas fraction is proportional to the square root of the SFE. The measured momentum injection due to thermal and radiation pressure forces is proportional to $Sigma_0^{-0.74}$, and the ejection of neutrals substantially contributes to the disruption of low-mass and/or high-surface density clouds. We present semi-analytic models for cloud dispersal mediated by photoevaporation and by dynamical mass ejection, and show that the predicted net SFE and mass loss efficiencies are consistent with the results of our numerical simulations.
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