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Register a new userConfirmation of the VeLLO L1148-IRS: Star Formation at very low (Column) Density
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J. Kauffmann
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We report the detection of a compact (of order 5 arcsec; about 1800 AU projected size) CO outflow from L1148-IRS. This confirms that this Spitzer source is physically associated with the nearby (about 325 pc) L1148 dense core. Radiative transfer modeling suggests an internal luminosity of 0.08 to 0.13 L_sun. This validates L1148-IRS as a Very Low Luminosity Object (VeLLO; L < 0.1 L_sun). The L1148 dense core has unusually low densities and column densities for a star-forming core. It is difficult to understand how L1148-IRS might have formed under these conditions. Independent of the exact final mass of this VeLLO (which is likely < 0.24 M_sun), L1148-IRS and similar VeLLOs might hold some clues about the isolated formation of brown dwarfs.
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We investigate the nature of the star formation law at low gas surface densities using a sample of 19 low surface brightness (LSB) galaxies with existing HI maps in the literature, UV imaging from the Galaxy Evolution Explorer satellite, and optical images from the Sloan Digital Sky Survey. All of the LSB galaxies have (NUV-r) colors similar to those for higher surface brightness star-forming galaxies of similar luminosity indicating that their average star formation histories are not very different. Based upon four LSB galaxies with both UV and FIR data, we find FIR/UV ratios significantly less than one, implying low amounts of internal UV extinction in LSB galaxies. We use the UV images and HI maps to measure the star formation rate and hydrogen gas surface densities within the same region for all of the galaxies. The LSB galaxy star formation rate surface densities lie below the extrapolation of the power law fit to the star formation rate surface density as a function of the total gas density for higher surface brightness galaxies. Although there is more scatter, the LSB galaxies also lie below a second version of the star formation law in which the star formation rate surface density is correlated with the gas density divided by the orbital time in the disk. The downturn seen in both star formation laws is consistent with theoretical models that predict lower star formation efficiencies in LSB galaxies due to the declining molecular fraction with decreasing density.
How do stars manage to form within low-density, HI-dominated gas? Such environments provide a laboratory for studying star formation with physical conditions distinct from starbursts and the metal-rich disks of spiral galaxies where most effort has been invested. Here we outline fundamental open questions about the nature of star formation at low-density. We describe the wide-field, high-resolution UV-optical-IR-radio observations of stars, star clusters and gas clouds in nearby galaxies needed in the 2020s to provide definitive answers, essential for development of a complete theory of star formation.
We present a simplified chemical and thermal model designed to allow computationally efficient study of the thermal evolution of metal-poor gas within large numerical simulations. Our main simplification is the neglect of the molecular chemistry of the heavy elements. The only molecular chemistry retained within the model is the formation and destruction of molecular hydrogen. Despite this major simplification, the model allows for accurate treatment of the thermal evolution of the gas within a large volume of parameter space. It is valid for temperatures 50 < T < 10000 K and metallicities 0 < Z < 0.1 Z_solar. In gas with a metallicity Z = 0.1 Z_solar, and in the absence of an incident ultraviolet radiation field, it is valid for hydrogen number densities n_H < 500 / t_char cm^-3, where t_char is the size in Myr of the characteristic physical timescale of interest in the problem. If Z << 0.1 Z_solar, or if a strong ultraviolet radiation field is present, then the model remains accurate up to significantly higher densities. We also discuss some possible applications of this model.
We present Atacama Large Millimeter/submillimeter Array (ALMA) gas and dust observations at band 7 (339~GHz: 0.89~mm) of the protoplanetary disk around a very low mass star ZZ~Tau~IRS with a spatial resolution of 0farcs25. The $^{12}$CO~$J=3rightarrow2$ position--velocity diagram suggests a dynamical mass of ZZ~Tau~IRS of $sim$0.1--0.3~$M_{sun}$. The disk has a total flux density of 273.9 mJy, corresponding to an estimated mass of 24--50~$M_oplus$ in dust. The dust emission map shows a ring at $r=$ 58~au and an azimuthal asymmetry at $r=$ jh{45}~au with a position angle of 135degr. The properties of the asymmetry, including radial width, aspect ratio, contrast, and contribution to the total flux, were found to be similar to the asymmetries around intermediate mass stars ($sim$2~$M_{sun}$) such as MWC~758 and IRS~48. This implies that the asymmetry in the ZZ~Tau~IRS disk shares a similar origin with others, despite the star being $sim$10 times less massive. Our observations also suggest that the inner and outer parts of the disk may be misaligned. Overall, the ZZ~Tau~IRS disk shows evidence of giant planet formation at $sim$10 au scale at a few Myr. If confirmed, it will challenge existing core accretion models, in which such planets have been predicted to be extremely hard to form around very low mass stars.
Star formation is known to occur more readily where more raw materials are available. This is often expressed by a Kennicutt-Schmidt relation where the surface density of Young Stellar Objects (YSOs) is proportional to column density to some power, $mu$. The aim of this work was to determine if column density alone is sufficient to explain the locations of Class 0/I YSOs within Serpens South, Serpens Core, Ophiuchus, NGC1333 and IC348, or if there is clumping or avoidance that would point to additional influences on the star formation. Using the O-ring test as a summary statistic, 95 per cent confidence envelopes were produced for different values of $mu$ from probability models made using the Herschel column density maps. The YSOs were tested against four distribution models: the best-estimate of $mu$ for the region, $mu=0$ above a minimum column density threshold and zero probability elsewhere, $mu=1$, and the power-law that best represents the five regions as a collective, $mu=2.05 pm 0.20$. Results showed that $mu=2.05$ model was consistent with the majority of regions and, for those regions, the spatial distribution of YSOs at a given column density is consistent with being random. Serpens South and NGC1333 rejected the $mu = 2.05$ model on small scales of $sim 0.15 mathrm{pc}$ which implies that small-scale interactions may be necessary to improve the model. On scales above 0.15 pc, the positions of YSOs in all five regions can be well described using column density alone.
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