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

Far-Ultraviolet Activity Levels of F, G, K, and M dwarf Exoplanet Host Stars

We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs with the goals of characterizing their range of FUV activity levels, calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet (EUV) stellar flux, and investigating the potential for FUV emission lines to probe star-planet interactions (SPIs). We build this emission line sample from a combination of new and archival observations with the Hubble Space Telescope-COS and -STIS instruments, targeting the chromospheric and transition region emission lines of Si III, N V, C II, and Si IV. We find that the exoplanet host stars, on average, display factors of 5 - 10 lower UV activity levels compared with the non-planet hosting sample; this is explained by a combination of observational and astrophysical biases in the selection of stars for radial-velocity planet searches. We demonstrate that UV activity-rotation relation in the full F - M star sample is characterized by a power-law decline (with index $\alpha$ ~ -1.1), starting at rotation periods >~3.5 days. Using N V or Si IV spectra and a knowledge of the star's bolometric flux, we present a new analytic relationship to estimate the intrinsic stellar EUV irradiance in the 90 - 360 Ang band with an accuracy of roughly a factor of ~2. Finally, we study the correlation between SPI strength and UV activity in the context of a principal component analysis that controls for the sample biases. We find that SPIs are not a statistically significant contributor to the observed UV activity levels.Comment: ApJS, accepted. 33 pages in emulateapj, 13 figures, 10 table

Bench-to-bedside review: Latest results in hemorrhagic shock

Hemorrhagic shock is a leading cause of death in trauma patients worldwide. Bleeding control, maintenance of tissue oxygenation with fluid resuscitation, coagulation support, and maintenance of normothermia remain mainstays of therapy for patients with hemorrhagic shock. Although now widely practised as standard in the USA and Europe, shock resuscitation strategies involving blood replacement and fluid volume loading to regain tissue perfusion and oxygenation vary between trauma centers; the primary cause of this is the scarcity of published evidence and lack of randomized controlled clinical trials. Despite enormous efforts to improve outcomes after severe hemorrhage, novel strategies based on experimental data have not resulted in profound changes in treatment philosophy. Recent clinical and experimental studies indicated the important influences of sex and genetics on pathophysiological mechanisms after hemorrhage. Those findings might provide one explanation why several promising experimental approaches have failed in the clinical arena. In this respect, more clinically relevant animal models should be used to investigate pathophysiology and novel treatment approaches. This review points out new therapeutic strategies, namely immunomodulation, cardiovascular maintenance, small volume resuscitation, and so on, that have been introduced in clinics or are in the process of being transferred from bench to bedside. Control of hemorrhage in the earliest phases of care, recognition and monitoring of individual risk factors, and therapeutic modulation of the inflammatory immune response will probably constitute the next generation of therapy in hemorrhagic shock. Further randomized controlled multicenter clinical trials are needed that utilize standardized criteria for enrolling patients, but existing ethical requirements must be maintained

Activated partial thromboplastin time waveform analysis as specific sepsis marker in cardiopulmonary bypass surgery

Throughout the last years, several new diagnostic biomarkers have been introduced into clinical routine to identify a systemic inflammatory response syndrome (SIRS) or a septic state and to discriminate between these two entities. According to studies in selected patients, measurement of these biomarkers may be advantageous under certain clinical conditions. On an individual basis, however, these sepsis markers usually lack an adequate negative or positive predictive power. Therefore, physicians in charge still have to rely on a combination of personal experience and results from clinical or laboratory tests when deciding on a patient's therapy. For surgical patients, a key problem consists of the time delay which is associated with the diagnosis of serious postoperative infections and which may negatively affect outcome. It is in this context where the activated partial thromboplastin time waveform analysis may represent a promising new method to discriminate between SIRS and sepsis, thereby shortening the time to therapy. Nevertheless, studies involving large patient populations will be necessary to prove the efficacy of this new diagnostic concept either as a single tool or in combination with the measurement of other biomarkers

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, N₂). Semi-forbidden C II] λ2325 and UV-fluorescent H₂ emission were also measured to constrain inner disk populations of C⁺ and vibrationally excited H2. We find a significant positive correlation between 14 μm HCN emission and fluxes from the FUV continuum and C II] λ2325, consistent with model predictions requiring N₂ 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-H₂ 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