1,974 research outputs found
Pre main sequence: Accretion & Outflows
Low-mass pre-main sequence (PMS) stars are strong X-ray sources, because they
possess hot corona like their older main-sequence counterparts. Unique to young
stars, however, are X-rays from accretion and outflows, and both processes are
of pivotal importance for star and planet formation. We describe how X-ray data
provide important insight into the physics of accretion and outflows. First,
mass accreted from a circumstellar disk onto the stellar surface reaches
velocities up to a few hundred km/s, fast enough to generate soft X-rays in the
post-shock region of the accretion shock. X-ray observations together with
laboratory experiments and numerical simulations show that the accretion
geometry is complex in young stars. Specifically, the center of the accretion
column is likely surrounded by material shielding the inner flow from view but
itself also hot enough to emit X-rays. Second, X-rays are observed in two
locations of protostellar jets: an inner stationary emission component probably
related to outflow collimation and outer components, which evolve withing years
and are likely related to working surfaces where the shock travels through the
jet. Jet-powered X-rays appear to trace the fastest jet component and provide
novel information on jet launching in young stars. We conclude that X-ray data
will continue to be highly important for understanding star and planet
formation, because they directly probe the origin of many emission features
studied in other wavelength regimes. In addition, future X-ray missions will
improve sensitivity and spectral resolution to probe key model parameters (e.g.
velocities) in large samples of PMS stars.Comment: Invited chapter for the "Handbook of X-ray and Gamma-ray
Astrophysics" (Eds. C. Bambi and A. Santangelo, Springer Nature, 2022),
accepted (34 pages, 11 figures
Probing UV-sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
The UV radiation field is a critical regulator of gas-phase chemistry in surface layers of disks around young stars. In an effort to understand the relationship between photocatalyzing UV radiation fields and gas emission observed at infrared and submillimeter wavelengths, we present an analysis of new and archival Hubble Space Telescope (HST), Spitzer, ALMA, IRAM, and SMA data for five targets in the Lupus cloud complex and 14 systems in Taurus-Auriga. The HST spectra were used to measure Lyα and far-UV (FUV) continuum fluxes reaching the disk surface, which are responsible for dissociating relevant molecular species (e.g., HCN, Nâ). Semi-forbidden C II] λ2325 and UV-fluorescent Hâ emission were also measured to constrain inner disk populations of Câș and vibrationally excited H2. We find a significant positive correlation between 14 ÎŒm HCN emission and fluxes from the FUV continuum and C II] λ2325, consistent with model predictions requiring Nâ photodissociation and carbon ionization to trigger the main CN/HCN formation pathways. We also report significant negative correlations between submillimeter CN emission and both C II] and FUV continuum fluxes, implying that CN is also more readily dissociated in disks with stronger FUV irradiation. No clear relationships are detected between either CN or HCN and Lyα or UV-Hâ emission. This is attributed to the spatial stratification of the various molecular species, which span several vertical layers and radii across the inner and outer disk. We expect that future observations with the James Webb Space Telescope will build on this work by enabling more sensitive IR surveys than were possible with Spitzer
Probing UV-sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
The UV radiation field is a critical regulator of gas-phase chemistry in surface layers of disks around young stars. In an effort to understand the relationship between photocatalyzing UV radiation fields and gas emission observed at infrared and submillimeter wavelengths, we present an analysis of new and archival Hubble Space Telescope (HST), Spitzer, ALMA, IRAM, and SMA data for five targets in the Lupus cloud complex and 14 systems in Taurus-Auriga. The HST spectra were used to measure Lyα and far-UV (FUV) continuum fluxes reaching the disk surface, which are responsible for dissociating relevant molecular species (e.g., HCN, Nâ). Semi-forbidden C II] λ2325 and UV-fluorescent Hâ emission were also measured to constrain inner disk populations of Câș and vibrationally excited H2. We find a significant positive correlation between 14 ÎŒm HCN emission and fluxes from the FUV continuum and C II] λ2325, consistent with model predictions requiring Nâ photodissociation and carbon ionization to trigger the main CN/HCN formation pathways. We also report significant negative correlations between submillimeter CN emission and both C II] and FUV continuum fluxes, implying that CN is also more readily dissociated in disks with stronger FUV irradiation. No clear relationships are detected between either CN or HCN and Lyα or UV-Hâ emission. This is attributed to the spatial stratification of the various molecular species, which span several vertical layers and radii across the inner and outer disk. We expect that future observations with the James Webb Space Telescope will build on this work by enabling more sensitive IR surveys than were possible with Spitzer
Network of topological nodal planes, multifold degeneracies, and Weyl points in CoSi
We report the identification of symmetry-enforced nodal planes (NPs) in CoSi
providing the missing topological charges in an entire network of
band-crossings comprising in addition multifold degeneracies and Weyl points,
such that the fermion doubling theorem is satisfied. In our study we have
combined measurements of Shubnikov-de Haas (SdH) oscillations in CoSi with
material-specific calculations of the electronic structure and Berry curvature,
as well as a general analysis of the band topology of space group (SG) 198. The
observation of two nearly dispersionless SdH frequency branches provides
unambiguous evidence of four Fermi surface sheets at the R point that reflect
the symmetry-enforced orthogonality of the underlying wave functions at the
intersections with the NPs. Hence, irrespective of the spin-orbit coupling
strength, SG198 features always six- and fourfold degenerate crossings at R and
that are intimately connected to the topological charges distributed
across the network
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