Stars form in messy and chaotic environments within Giant Molecular Clouds (GMCs). These GMCs are very interesting objects because of the rich physical processes that describe their evolution – GMCs are magnetized, supersonically turbulent fluids which are altered by feedback from newly formed stars. Young stars produce energetic outflows, winds and carve bubbles of hot ionized gas. The most massive stars end their lives by going supernova, while new stars are still forming. All these physical processes feed gas back into the parent GMC and disrupt them.
Giant Molecular Clouds (GMCs), the sites of star formation in our Universe, are supersonically turbulent and self-gravitating. Understanding supersonic turbulence under the influence of self-gravity is therefore crucial to understanding the dynamics of GMCs and star formation. In Khullar et al (2021), we explored the density probability distribution function (PDF) produced in supersonic, isothermal, self-gravitating turbulence of the sort that is ubiquitous in star-forming molecular clouds. Our experiments cover a wide range of turbulent Mach number and virial parameter, allowing us for the first time to determine how the PDF responds as these parameters vary, and we introduce a new diagnostic, the dimensionless star formation efficiency versus density curve, which provides a sensitive diagnostic of the PDF shape and dynamics. We show that the PDF follows a universal functional form consisting of a lognormal at low density with two distinct power-law tails at higher density: the first of these represents the onset of self-gravitation, and the second reflects the onset of rotational support. Once the star formation efficiency reaches a few per cent, the PDF becomes statistically steady, with no evidence for secular time evolution at star formation efficiencies from about 5 to 20 per cent. We show that both the Mach number and the virial parameter influence the characteristic densities at which the lognormal gives way to the first power law, and the first to the second, and we extend (for the former) and develop (for the latter) simple theoretical models for the relationship between these density thresholds and the global properties of the turbulent medium. we explore the role of stellar feedback in the evolution of Giant Molecular Clouds.
In Khullar et al (2019), we examined claims of star formation thresholds in observations. Most gas in giant molecular clouds is relatively low-density and forms star inefficiently, converting only a small fraction of its mass to stars per dynamical time. However, star formation models generally predict the existence of a threshold density above which the process is efficient and most mass collapses to stars on a dynamical timescale. A number of authors have proposed observational techniques to search for a threshold density above which star formation is efficient, but it is unclear which of these techniques, if any, are reliable. In this work we used detailed simulations of turbulent, magnetised star-forming clouds, including stellar radiation and outflow feedback, to investigate whether it is possible to recover star formation thresholds using current observational techniques. Using mock observations of the simulations at realistic resolutions, we show that plots of projected star formation efficiency per free-fall time εff can detect the presence of a threshold, but that the resolutions typical of current dust emission or absorption surveys are insufficient to determine its value. In contrast, proposed alternative diagnostics based on a change in the slope of the gas surface density versus star formation rate surface density (Kennicutt-Schmidt relation) or on the correlation between young stellar object counts and gas mass as a function of density are ineffective at detecting thresholds even when they are present. The signatures in these diagnostics is sometimes taken as indicative of a threshold in observations, which we generally reproduce in our mock observations, do not prove to correspond to real physical features in the 3D gas distribution.
In the past, I’ve been interested in understanding He re-ionization. Under the supervision of Prof. Benedetta Ciardi (MPA, Garching), I looked at the hyperfine transition of 3He+ as a probe of the high-z universe. The hyperfine transition of 3He+ at 3.5 cm has been thought as a probe of the high-z IGM since it offers a unique insight into the evolution of the helium component of the gas, as well as potentially give an independent constraint on the 21 cm signal from neutral hydrogen. In Khullar et al (2020), we use radiative transfer simulations of reionization driven by sources such as stars, X-ray binaries, accreting black holes and shock heated interstellar medium, and simulations of a high-z quasar to characterize the signal and analyze its prospects of detection. We find that the peak of the signal lies in the range ∼ 1 − 50 μK for both environments, but while around the quasar it is always inemission, in the case of cosmic reionization a brief period of absorption is expected. As the evolution of HeII is determined by stars, we find that it is not possible to distinguish reionization histories driven by more energetic sources. On the other hand, while a bright QSO produces a signal in 21 cm that is very similar to the one from a large collection of galaxies, its signature in 3.5 cm is very peculiar and could be a powerful probe to identify the presence of the QSO. We analyze the prospects of the signal’s detectability using SKA1-mid as our reference telescope. We find that the noise power spectrum dominates over the power spectrum of the signal, although the S/N ratio can be appreciable when the wavenumber bin width and the survey volume are sufficiently large.
