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Register a new userObservational signatures of outbursting protostars - I: From hydrodynamic simulations to observations
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Dimitris Stamatellos
Publication date
2019
fields
Physics
and research's language is
English
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Accretion onto protostars may occur in sharp bursts. Accretion bursts during the embedded phase of young protostars are probably most intense, but can only be inferred indirectly through long-wavelength observations. We perform radiative transfer calculations for young stellar objects (YSOs) formed in hydrodynamic simulations to predict the long wavelength, sub-mm and mm, flux responses to episodic accretion events, taking into account heating from the young protostar and from the interstellar radiation field. We find that the flux increase due to episodic accretion events is more prominent at sub-mm wavelengths than at mm wavelengths; e.g. a factor of ~570 increase in the luminosity of the young protostar leads to a flux increase of a factor of 47 at 250 micron but only a factor of 10 at 1.3 mm. Heating from the interstellar radiation field may reduce further the flux increase observed at longer wavelengths. We find that during FU Ori-type outbursts the bolometric temperature and luminosity may incorrectly classify a source as a more evolved YSO, due to a larger fraction of the radiation of the object being emitted at shorter wavelengths
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Young stars exhibit variability due to changes in the gas accretion rate onto them, an effect that should be quite significant in the early stages of their formation. As protostars are embedded within their natal cloud, this variability may only be inferred through long wavelength observations. We perform radiative transfer simulations of young stellar objects (YSOs) formed in hydrodynamical simulations, varying the structure and luminosity properties in order to estimate the long-wavelength, sub-mm and mm, variations of their flux. We find that the flux increase due to an outburst event depends on the protostellar structure and is more prominent at sub-mm wavelengths than at mm wavelengths; e.g. a factor of 40 increase in the luminosity of the young protostar leads to a flux increase of a factor of 10 at 250 micron but only a factor of 2.5 at 1.3 mm. We find that the interstellar radiation field dilutes the flux increase but that this effect may be avoided if resolution permits the monitoring of the inner regions of a YSO, where the heating is primarily due to protostellar radiation. We also confirm that the bolometric temperature and luminosity of outbursting protostars may result in an incorrect classification of their evolutionary stage.
We present results from three-dimensional, self-gravitating, radiation-hydrodynamic simulations of low-mass protostellar outflows. We construct synthetic observations in 12CO in order to compare with observed outflows and evaluate the effects of beam resolution and outflow orientation on inferred outflow properties. To facilitate the comparison, we develop a quantitative prescription for measuring outflow opening angles. Using this prescription, we demonstrate that, in both simulations and synthetic observations, outflow opening angles broaden with time similarly to observed outflows. However, the interaction between the outflowing gas and the turbulent core envelope produces significant asymmetry between the red and blue shifted outflow lobes. We find that applying a velocity cutoff may result in outflow masses that are underestimated by a factor 5 or more, and masses derived from optically thick CO emission further underpredict the mass of the high-velocity gas by a factor of 5-10. Derived excitation temperatures indicate that outflowing gas is hotter than the ambient gas with temperature rising over time, which is in agreement with the simulation gas temperatures. However, excitation temperatures are otherwise not well correlated with the actual gas temperature.
Through synthetic observations of a hydrodynamical simulation of an evolving star-forming region, we assess how the choice of observational techniques affects the measurements of properties which trace star formation. Testing and calibrating observational measurements requires synthetic observations which are as realistic as possible. In this part of the paper series (Paper I), we explore different techniques for how to map the distributions of densities and temperatures from the particle-based simulations onto a Voronoi mesh suitable for radiative transfer and consequently explore their accuracy. We further test different ways to set up the radiative transfer in order to produce realistic synthetic observations. We give a detailed description of all methods and ultimately recommend techniques. We have found that the flux around 20 microns is strongly overestimated when blindly coupling the dust radiative transfer temperature with the hydrodynamical gas temperature. We find that when instead assuming a constant background dust temperature in addition to the radiative transfer heating, the recovered flux is consistent with actual observations. We present around 5800 realistic synthetic observations for Spitzer and Herschel bands, at different evolutionary time-steps, distances and orientations. In the upcoming papers of this series (Paper II, Paper III and Paper IV), we will test and calibrate measurements of the star-formation rate (SFR), gas mass and the star-formation efficiency (SFE) using our realistic synthetic observations.
During these last decades, our knowledge of evolutionary and structural properties of stars of different mass and chemical composition is significantly improved. This result has been achieved as a consequence of our improved capability in understanding and describing the physical behavior of matter in the different thermal regimes characteristic of the various stellar mass ranges and evolutionary stages. This notwithstanding, current generation of stellar models is still affected by significant and, usually, not negligible uncertainties. These uncertainties are related to our poor knowledge of some physical proceses occurring in the real stars such as, for instance, some thermodynamical processes, nuclear reaction rates, as well as the efficiency of mixing processes. These drawbacks of stellar models have to be properly taken into account when comparing theory with observations in order to derive relevant information about the properties of both resolved and unresolved stellar populations. On the other hand, observations of both field and cluster stars can provide fundamental benchmarks for constraining the reliability and accuracy of the theoretical framework. In the following we review some important evolutionary and structural properties of very-low and low-mass stars, as well as the most important uncertainties affecting the stellar models for such stars. We show what are the main sources of uncertainty along the main evolutionary stages, and discuss the present level of agreement between theory and observations.
We present a detailed study of scaling relations between total cluster mass and three mass proxies based on X-ray observables: temperature of the intra-cluster medium, gas mass and the product of the two, Y_X. Our analysis is based on two sets of high-resolution hydrodynamical simulations performed with the TreePM-SPH GADGET code. The first set includes about 140 clusters with masses above 5x10^13 M_sun/h (30 having mass above 10^15 M_sun/h), that have been simulated with (i) non-radiative physics and including (ii) cooling, star formation, chemical enrichment and the effect of supernova feedback triggering galactic ejecta. This large statistics is used to quantify the robustness of the scaling relations, to determine their redshift evolution and to calibrate their intrinsic scatter and its distribution. We use a smaller set of clusters including 18 halos with masses above 5x10^13 M_sun/h to test the robustness of mass proxies against changing the physical processes included in simulations (thermal conduction, artificial viscosity, cooling and star formation, galactic winds and AGN feedback). We find the M-Y_X scaling relation to be the least sensitive one to variations of the ICM physics, with its slope and redshift evolution close to the self-similar model predictions. The distribution of the scatter around the best-fitting relations is close to a log-normal one. M_gas has the smallest scatter in mass, with values of sigma_lnM = 0.04-0.06, depending on the physics included in the simulation, and with a mild dependence on redshift. The M-T relation is the one with the largest scatter, with sigma_lnM > 0.1 at z=0, increasing to > 0.15 at z=1. The intrinsic scatter in the M-Y_X relation is slightly larger than in the M-M_gas relation. These results confirm that both Y_X and M_gas mass proxies are well suited for cosmological applications of future large X-ray surveys. [abridged]
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