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Register a new userResolving numerical star formation: A cautionary tale
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Resolution studies of test problems set baselines and help define minimum resolution requirements, however, resolution studies must also be performed on scientific simulations to determine the effect of resolution on the specific scientific results. We perform a resolution study on the formation of a protostar by modelling the collapse of gas through 14 orders of magnitude in density. This is done using compressible radiative non-ideal magnetohydrodynamics. Our suite consists of an ideal magnetohydrodynamics (MHD) model and two non-ideal MHD models, and we test three resolutions for each model. The resulting structure of the ideal MHD model is approximately independent of resolution, although higher magnetic field strengths are realised in higher resolution models. The non-ideal MHD models are more dependent on resolution, specifically the magnetic field strength and structure. Stronger magnetic fields are realised in higher resolution models, and the evolution of detailed structures such as magnetic walls are only resolved in our highest resolution simulation. In several of the non-ideal MHD models, there is an off-set between the location of the maximum magnetic field strength and the maximum density, which is often obscured or lost at lower resolutions. Thus, understanding the effects of resolution on numerical star formation is imperative for understanding the formation of a star.
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The attenuation of light in star forming galaxies is correlated with a multitude of physical parameters including star formation rate, metallicity and total dust content. This variation in attenuation is even more prevalent on the kiloparsec scale, which is relevant to many current spectroscopic integral field unit surveys. To understand the cause of this variation, we present and analyse textit{Swift}/UVOT near-UV (NUV) images and SDSS/MaNGA emission-line maps of 29 nearby ($z<0.084$) star forming galaxies. We resolve kiloparsec-sized star forming regions within the galaxies and compare their optical nebular attenuation (i.e., the Balmer emission line optical depth, $tau^l_Bequivtau_{textrm{H}beta}-tau_{textrm{H}alpha}$) and NUV stellar continuum attenuation (via the NUV power-law index, $beta$) to the attenuation law described by Battisti et al. The data agree with that model, albeit with significant scatter. We explore the dependence of the scatter of the $beta$-$tau^l_B$ measurements from the star forming regions on different physical parameters, including distance from the nucleus, star formation rate and total dust content. Finally, we compare the measured $tau^l_B$ and $beta$ between the individual star forming regions and the integrated galaxy light. We find a strong variation in $beta$ between the kiloparsec scale and the larger galaxy scale not seen in $tau^l_B$. We conclude that the sight-line dependence of UV attenuation and the reddening of $beta$ due to the light from older stellar populations could contribute to the $beta$-$tau^l_B$ discrepancy.
We explore the pitfalls which affect the comparison of the star-formation (SF) relation for nearby molecular clouds with that for distant compact molecular clumps. We show that both relations behave differently in the ($Sigma_{gas}$, $Sigma_{SFR}$) space, where $Sigma_{gas}$ and $Sigma_{SFR}$ are, respectively, the gas and SF rate surface densities, even when the physics of star formation is the same. This is because the SF relation of nearby clouds relates gas and star surface densities measured locally, that is, within a given interval of gas surface density, or at a given protostar location. We refer to such measurements as local measurements, and the corresponding SF relation as the local relation. In contrast, the stellar content of a distant molecular clump remains unresolved. Only the mean SF rate can be obtained from e.g. the clump infrared luminosity. One clump therefore provides one single point to the ($Sigma_{gas}$, $Sigma_{SFR}$) space, that is, its mean gas surface density and SF rate surface density. We refer to this SF relation as a global relation since it builds on the global properties of molecular clumps. Its definition therefore requires an ensemble of cluster-forming clumps. We show that, although the local and global relations have different slopes, this per se cannot be taken as evidence for a change in the physics of SF with gas surface density. It therefore appears that great caution should be taken when physically interpreting a composite SF relation, that is, a relation combining together local and global measurements.
We present three different methods to estimate error bars on the predictions made using a neural network. All of them represent lower bounds for the extrapolation errors. For example, we did not include an analysis on robustness against small perturbations of the input data. At first, we illustrate the methods through a simple toy model, then, we apply them to some realistic cases related to nuclear masses. By using theoretical data simulated either with a liquid-drop model or a Skyrme energy density functional, we benchmark the extrapolation performance of the neural network in regions of the Segr`e chart far away from the ones used for the training and validation. Finally, we discuss how error bars can help identifying when the extrapolation becomes too uncertain and thus unreliable
Analysis of cluster and field star uvby data demonstrates the existence of a previously undetected discrepancy in a widely used photometric metallicity calibration for G dwarfs. The discrepancy is systematic and strongly color-dependent, reducing the estimated [Fe/H] for stars above [Fe/H] ~ -0.2 by between +0.1 and +0.4 dex, and creating a deficit of metal-rich stars among dwarfs of mid-G and later spectral type. The source of the problem, triggered for stars with b-y greater than about 0.47, appears to be an enhanced metallicity dependence for the c1 index that increases as temperature declines. The link between c1, normally a surface gravity indicator, and metallicity produces two secondary effects. The deficit in the photometric abundance for a cool dwarf is partially compensated by some degree of evolution off the main sequence and cool dwarfs with metallicities significantly above the Hyades are found to have c1 indices that classify them as giants. The potential impact of the problem on stellar population studies is discussed.
We present a comprehensive study of Z CVn, an RR Lyrae star that shows long-term cyclic variations of its pulsation period. A possible explanation suggested from the shape of the O-C diagram is the light travel-time effect, which we thoroughly examine. We used original photometric and spectroscopic measurements and investigated the period evolution using available maximum times spanning more than one century. If the binary hypothesis is valid, Z CVn orbits around a black hole with minimal mass of $56.5$ $mathfrak{M}_{odot}$ on a very wide ($P_{rm orbit}=78.3$ years) and eccentric orbit ($e=0.63$). We discuss the probability of a formation of a black hole-RR Lyrae pair and, although we found it possible, there is no observational evidence of the black hole in the direction to Z CVn. However, the main objection against the binary hypothesis is the comparison of the systemic radial velocity curve model and spectroscopic observations that clearly show that Z CVn cannot be bound in such a binary. Therefore, the variations of pulsation period are likely intrinsic to the star. This finding represents a discovery/confirmation of a new type of cyclic period changes in RR Lyrae stars. By the analysis of our photometric data, we found that the Blazhko modulation with period of 22.931 d is strongly dominant in amplitude. The strength of the phase modulation varies and is currently almost undetectable. We also estimated photometric physical parameters of Z CVn and investigated their variations during the Blazhko cycle using the Inverse Baade-Wesselink method.
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