Interpreting the sub-linear Kennicutt-Schmidt relationship: The case for diffuse molecular gas

Recent statistical analysis of two extragalactic observational surveys strongly indicate a sublinear Kennicutt-Schmidt (KS) relationship between the star formation rate (Sigsfr) and molecular gas surface density (Sigmol). Here, we consider the consequences of these results in the context of common assumptions, as well as observational support for a linear relationship between Sigsfr and the surface density of dense gas. If the CO traced gas depletion time (tau_mol) is constant, and if CO only traces star forming giant molecular clouds (GMCs), then the physical properties of each GMC must vary, such as the volume densities or star formation rates. Another possibility is that the conversion between CO luminosity and Sigmol, the XCO factor, differs from cloud-to-cloud. A more straightforward explanation is that CO permeates the hierarchical ISM, including the filaments and lower density regions within which GMCs are embedded. A number of independent observational results support this description, with the diffuse gas comprising at least 30% of the total molecular content. The CO bright diffuse gas can explain the sublinear KS relationship, and consequently leads to an increasing tau_mol with Sigmol. If Sigsfr linearly correlates with the dense gas surface density, a sublinear KS relationship indicates that the fraction of diffuse gas fdiff grows with Sigmol. In galaxies where Sigmol falls towards the outer disk, this description suggests that fdiff also decreases radially.Comment: 8 pages, 4 figures, to appear in MNRAS, comments welcom

On the temperature structure of the Galactic Centre cloud G0.253+0.016

We present a series of smoothed particle hydrodynamical models of G0.253+0.016 (also known as 'The Brick'), a very dense molecular cloud that lies close to the Galactic Centre. We explore how its gas and dust temperatures react as we vary the strength of both the interstellar radiation field (ISRF) and the cosmic ray ionisation rate (CRIR). As the physical extent of G0.253+0.016 along our line-of-sight is unknown, we consider two possibilities: one in which the longest axis is that measured in the plane of the sky (9.4 pc in length), and one in which it is along the line of sight, in which case we take it to be 17 pc. To recover the observed gas and dust temperatures, we find find that the ISRF must be around 1000 times the solar neighbourhood value, and the CRIR must be roughly 1E-14 /s, regardless of the geometries studied. For such high values of the CRIR, we find that cooling in the cloud's interior is dominated by neutral oxygen, in contrast to standard molecular clouds, which at the same densities are mainly cooled via CO. Our results suggest that the conditions near G0.253+0.016 are more extreme than those generally accepted for the inner 500 pc of the galaxy.Comment: 6 pages, 4 figures, 1 table, accepted for publication in ApJ Letter

Prediction of the CPCP asymmetry C00C_{00} in B0→D0D0‾B^0 \to D^0\overline{D^0} decay

Of all $B \to D \overline{D}$ decays, the $B^0 \to D^0 \overline{D^0}$ decay has the smallest observed branching ratio as it takes place primarily via the suppressed $W$-exchange diagram. The $CP$ asymmetry for this mode is yet to be measured experimentally. By exploiting the relationship among the decay amplitudes of $B \to D\overline{D}$ decays (using isospin and topological amplitudes) we are able to relate the $CP$ asymmetries and branching ratios by a simple expression. This enables us to predict the $CP$ asymmetry $C_{00}$ in $B^0 \to D^0 \overline{D^0}$. While the predicted central values of $C_{00}$ are outside the physically allowed region, they are currently associated with large uncertainties owing to the large errors in the measurements of the $B^0 \to D^0 \overline{D^0}$ branching ratio ($B_{00}$), the other $CP$ asymmetries $C_{+-}$ (of $B^0 \to D^+ D^-$) and $A_{\text{CP}}$ (of $B^+ \to D^+ \overline{D^0}$). With a precise determination of $B_{00}$, $C_{+-}$ and $A_{\text{CP}}$, one can use our analytical result to predict $C_{00}$ with a reduced error and compare it with the experimental measurement when it becomes available. The correlation between $B_{00}$ and $C_{00}$ is an interesting aspect that can be probed in ongoing and future particle physics experiments such as LHCb and Belle II.Comment: 21 pages, 6 figures, accepted for publication in JHE

Interaction Between Motor Domains Can Explain the Complex Dynamics of Heterodimeric Kinesins

Motor proteins are active enzyme molecules that play a crucial role in many biological processes. They transform the chemical energy into the mechanical work and move unidirectionally along rigid cytoskeleton filaments. Single-molecule experiments suggest that motor proteins, consisting of two motor domains, move in a hand-over-hand mechanism when each subunit changes between trailing and leading positions in alternating steps, and these subunits do not interact with each other. However, recent experiments on heterodimeric kinesins suggest that the motion of motor domains is not independent, but rather strongly coupled and coordinated, although the mechanism of these interactions are not known. We propose a simple discrete stochastic model to describe the dynamics of homodimeric and heterodimeric two-headed motor proteins. It is argued that interactions between motor domains modify free energy landscapes of each motor subunit, and motor proteins still move via the hand-over-hand mechanism but with different transitions rates. Our calculations of biophysical properties agree with experimental observations. Several ways to test the theoretical model are proposed.Comment: To appear in New J. Phy

Indications of a sub-linear and non-universal Kennicutt-Schmidt relationship

We estimate the parameters of the Kennicutt-Schmidt (KS) relationship, linking the star formation rate (Sigma_SFR) to the molecular gas surface density (Sigma_mol), in the STING sample of nearby disk galaxies using a hierarchical Bayesian method. This method rigorously treats measurement uncertainties, and provides accurate parameter estimates for both individual galaxies and the entire population. Assuming standard conversion factors to estimate Sigma_SFR and Sigma_mol from the observations, we find that the KS parameters vary between galaxies, indicating that no universal relationship holds for all galaxies. The KS slope of the whole population is 0.76, with the 2sigma range extending from 0.58 to 0.94. These results imply that the molecular gas depletion time is not constant, but varies from galaxy to galaxy, and increases with the molecular gas surface density. Therefore, other galactic properties besides just Sigma_mol affect Sigma_SFR, such as the gas fraction or stellar mass. The non-universality of the KS relationship indicates that a comprehensive theory of star formation must take into account additional physical processes that may vary from galaxy to galaxy.Comment: 7 pages, 2 figures, 1 table. Updated to match MNRAS accepted versio

Electronic paddle-wheels in a solid-state electrolyte

Solid-state superionic conductors (SSICs) are promising alternatives to liquid electrolytes in batteries and other energy storage technologies. The rational design of SSICs and ultimately their deployment in battery technologies is hindered by the lack of a thorough understanding of their ion conduction mechanisms. In SSICs containing molecular ions, rotational dynamics couple to translational diffusion to create a 'paddle-wheel' effect that facilitates conduction. The paddle-wheel mechanism explains many important features of molecular SSICs, but an explanation for ion conduction and anharmonic lattice dynamics in SSICs composed of monatomic ions is still needed. We predict that ion conduction in the classic SSIC AgI involves 'electronic paddle-wheels,' rotational motion of lone pairs that couple to and facilitate ion diffusion. The electronic paddle-wheel mechanism creates a universal perspective for understanding ion conductivity in both monatomic and molecular SSICs that will create design principles for engineering solid-state electrolytes from the electronic level up to the macroscale.Comment: 6 pages, 3 figure