Behavior of a polymer chain in a critical binary solvent

We present a field-theoretic renormalization group analysis of a polymer chain immersed in a binary good solvent close to its critical demixing point. We first show that this problem can be mapped on a bicritical field theory, i.e. a $(\Phi^{2})^{2}$-model with a mass anisotropy. This implies that the end-to-end distance of the polymer is now controlled by a new critical exponent $\nu_{B}$ related to the quadratic mass anisotropy operator $B$. To show this we solve the RG equation and calculate explicitly the exponents and the mean end-to-end length of the chain.Comment: 6 pages, accepted in Europhys. Let

The scaling behaviour of screened polyelectrolytes

We present a field-theoretic renormalization group (RG) analysis of a single flexible, screened polyelectrolyte chain (a Debye-H\"uckel chain) in a polar solvent. We point out that the Debye-H\"uckel chain may be mapped onto a local field theory which has the same fixed point as a generalised $n \to 1$ Potts model. Systematic analysis of the field theory shows that the system is one with two interplaying length-scales requiring the calculation of scaling functions as well as exponents to fully describe its physical behaviour. To illustrate this, we solve the RG equation and explicitly calculate the exponents and the mean end-to-end length of the chain.Comment: 6 pages, 1 figure; changed title and slight modification to tex

A dichotomy in group II Herbig disks: ALMA gas disk height measurements show both shadowed large vertically extended disks and compact flat disks

Herbig stars can be classified into group I and group II depending on the shape of the far-IR excess from the spectral energy distribution. This separation may be evolutionary and related to the vertical structure of these disks. We aim to determine the emission height of Herbig disks and compare the resulting vertical extent of both groups. ALMA Band 6 observations of 12CO emission lines at sufficient velocity and spatial resolution of eight Herbig disks (four group I and four group II sources) are used to determine the emission heights from the channel maps via geometrical methods developed in other works. We find that all group I disks are vertically extended with a height to radius ratio of at least 0.25, and for three of the disks the gas emission profile can be traced out to 200-500 au. The group II disks are divided between MWC 480 and HD 163296 which have similar emission height profiles as the group I disks, and AK Sco and HD 142666 which are very flat (not exceeding a height of 10 au) and more compact (<200 au in size). The brightness temperatures show no differences between the disks when the luminosity of the host star is accounted for. Our findings agree with previous work suggesting that group I disks are vertically extended and that group II disks are either large and self-shadowed or compact. Both MWC 480 and HD 163296 could be precursors of group I disks, which we see now before a cavity has formed that would allow irradiation of the outer parts of the disk. The very flat disks AK Sco and HD 142666 could be due to significant settling because of the advanced age of these disks (~20 instead of <10 Myr). These large differences in vertical structures are not reflected in the spectral energy distributions of these disks. More and deeper observations at higher spatial and velocity resolution are necessary to further characterize the Herbig sub-groups.Comment: Accepted for publication in Astronomy and Astrophysics. 8 pages, 4 figures, plus appendice

The <i>M^.–M</i>_disk Relationship for Herbig Ae/Be Stars: A lifetime problem for disks with low masses?

The accretion of material from protoplanetary disks onto their central stars is a fundamental process in the evolution of these systems and a key diagnostic in constraining the disk lifetime. We analyze the relationship between the stellar accretion rate and the disk mass in 32 intermediate-mass Herbig Ae/Be systems and compare them to their lower-mass counterparts, T Tauri stars. We find that the M ̇-Mdisk relationship for Herbig Ae/Be stars is largely flat at ∼10−7 M☉ yr−1 over 3 orders of magnitude in dust mass. While most of the sample follows the T Tauri trend, a subset of objects with high accretion rates and low dust masses are identified. These outliers (12 out of 32 sources) have an inferred disk lifetime of less than 0.01 Myr and are dominated by objects with low infrared excess. This outlier sample is likely identified in part by the bias in classifying Herbig Ae/Be stars, which requires evidence of accretion that can only be reliably measured above a rate of ∼10−9M☉ yr−1 for these spectral types. If the disk masses are not underestimated and the accretion rates are not overestimated, this implies that these disks may be on the verge of dispersal, which may be due to efficient radial drift of material or outer disk depletion by photoevaporation and/or truncation by companions. This outlier sample likely represents a small subset of the larger young, intermediate-mass stellar population, the majority of which would have already stopped accreting and cleared their disks

The M˙\dot{M}--MdiskM_{\rm{disk}} relationship for Herbig Ae/Be stars: a lifetime problem for disks with low masses?

The accretion of material from protoplanetary disks onto their central stars is a fundamental process in the evolution of these systems and a key diagnostic in constraining the disk lifetime. We analyze the relationship between the stellar accretion rate and the disk mass in 32 intermediate-mass Herbig Ae/Be systems and compare them to their lower-mass counterparts, T Tauri stars. We find that the $\dot{M}$--$M_{\rm{disk}}$ relationship for Herbig Ae/Be stars is largely flat at $\sim$10$^{-7}$ M$_{\odot}$ yr$^{-1}$ across over three orders of magnitude in dust mass. While most of the sample follows the T Tauri trend, a subset of objects with high accretion rates and low dust masses are identified. These outliers (12 out of 32 sources) have an inferred disk lifetime of less than 0.01 Myr and are dominated by objects with low infrared excess. This outlier sample is likely identified in part by the bias in classifying Herbig Ae/Be stars, which requires evidence of accretion that can only be reliably measured above a rate of $\sim$10$^{-9}$ M$_{\odot}$ yr$^{-1}$ for these spectral types. If the disk masses are not underestimated and the accretion rates are not overestimated, this implies that these disks may be on the verge of dispersal, which may be due to efficient radial drift of material or outer disk depletion by photoevaporation and/or truncation by companions. This outlier sample likely represents a small subset of the larger young, intermediate-mass stellar population, the majority of which would have already stopped accreting and cleared their disks.Comment: 16 pages, 3 figures, accepted to A