Alan Aversa | University of Arizona (original) (raw)
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Papers by Alan Aversa
We present a combined analysis of the low-mass Initial Mass Function (IMF) for seven star forming... more We present a combined analysis of the low-mass Initial Mass Function (IMF) for seven star forming regions. We first demonstrate that the ratios of stars to brown dwarfs are consistent with a single underlying IMF. Assuming the underlying IMF is the same for all seven clusters and by combining the ratio of stars to brown dwarfs from each cluster we constrain the shape of the brown dwarf IMF and find it to be consistent with a log--normal IMF. This provides the strongest constraint yet that the sub-stellar IMF turns over (dN/dM M^(-alpha), alpha < 0).
In order to investigate the relationship between the local environment and the properties of nata... more In order to investigate the relationship between the local environment and the properties of natal star clusters, we obtained radio observations of 25 star-forming galaxies within 20 Mpc using the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). Natal star-forming regions can be identified by their characteristic thermal radio emission, which is manifest in their spectral index at centimeter wavelengths. The host galaxies in our sample were selected based upon their likelihood of harboring young star formation. In star-forming regions, the ionizing flux of massive embedded stars powers the dominant thermal free-free emission of those sources, resulting in a spectral index of {\alpha} {\gtrsim} -0.2 (where S{\nu} {\propto} {\nu}{\alpha}), which we compute. With the current sensitivity, we find that of the 25 galaxies in this sample only five have radio sources with spectral indices that are only consistent with a thermal origin; four have radio sources that are only consistent with a non-thermal origin; six have radio sources whose nature is ambiguous due to uncertainties in the spectral index; and sixteen have no detected radio sources. For those sources that appear to be dominated by thermal emission, we infer the ionizing flux of the star clusters and the number of equivalent O7.5 V stars that are required to produce the observed radio flux densities. The most radio-luminous clusters that we detect have an equivalent of ~7x103 O7.5 V stars, and the smallest only have an equivalent of ~102 O7.5 V stars; thus these star-forming regions span the range of large OB-associations to moderate "super star clusters" (SSCs). With the current detection limits, we also place upper limits on the masses of clusters that could have recently formed...
Astrophysical Journal, Jan 1, 2008
We present a combined analysis of the low-mass initial mass function (IMF) for seven star-forming... more We present a combined analysis of the low-mass initial mass function (IMF) for seven star-forming regions. We first demonstrate that the ratios of stars to brown dwarfs are consistent with a single underlying IMF. By assuming that the underlying IMF is the same for all seven clusters and by combining the ratio of stars to brown dwarfs from each cluster we constrain the shape of the brown dwarf IMF and find it to be consistent with a lognormal IMF . This provides the strongest constraint yet that the substellar IMF turns over ( dN dM ∝ M −α , α < 0).
We present a combined analysis of the low-mass Initial Mass Function (IMF) for seven star forming... more We present a combined analysis of the low-mass Initial Mass Function (IMF) for seven star forming regions. We first demonstrate that the ratios of stars to brown dwarfs are consistent with a single underlying IMF. Assuming the underlying IMF is the same for all seven clusters and by combining the ratio of stars to brown dwarfs from each cluster we constrain the shape of the brown dwarf IMF and find it to be consistent with a log--normal IMF. This provides the strongest constraint yet that the sub-stellar IMF turns over (dN/dM M^(-alpha), alpha < 0).
In order to investigate the relationship between the local environment and the properties of nata... more In order to investigate the relationship between the local environment and the properties of natal star clusters, we obtained radio observations of 25 star-forming galaxies within 20 Mpc using the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). Natal star-forming regions can be identified by their characteristic thermal radio emission, which is manifest in their spectral index at centimeter wavelengths. The host galaxies in our sample were selected based upon their likelihood of harboring young star formation. In star-forming regions, the ionizing flux of massive embedded stars powers the dominant thermal free-free emission of those sources, resulting in a spectral index of {\alpha} {\gtrsim} -0.2 (where S{\nu} {\propto} {\nu}{\alpha}), which we compute. With the current sensitivity, we find that of the 25 galaxies in this sample only five have radio sources with spectral indices that are only consistent with a thermal origin; four have radio sources that are only consistent with a non-thermal origin; six have radio sources whose nature is ambiguous due to uncertainties in the spectral index; and sixteen have no detected radio sources. For those sources that appear to be dominated by thermal emission, we infer the ionizing flux of the star clusters and the number of equivalent O7.5 V stars that are required to produce the observed radio flux densities. The most radio-luminous clusters that we detect have an equivalent of ~7x103 O7.5 V stars, and the smallest only have an equivalent of ~102 O7.5 V stars; thus these star-forming regions span the range of large OB-associations to moderate "super star clusters" (SSCs). With the current detection limits, we also place upper limits on the masses of clusters that could have recently formed...
Astrophysical Journal, Jan 1, 2008
We present a combined analysis of the low-mass initial mass function (IMF) for seven star-forming... more We present a combined analysis of the low-mass initial mass function (IMF) for seven star-forming regions. We first demonstrate that the ratios of stars to brown dwarfs are consistent with a single underlying IMF. By assuming that the underlying IMF is the same for all seven clusters and by combining the ratio of stars to brown dwarfs from each cluster we constrain the shape of the brown dwarf IMF and find it to be consistent with a lognormal IMF . This provides the strongest constraint yet that the substellar IMF turns over ( dN dM ∝ M −α , α < 0).