The impact of the metallicity and star formation rate on the time-dependent galaxy-wide stellar initial mass function

The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses being represented by the canonical IMF. As a consequence the galaxy-wide IMF (gwIMF), defined as the sum of the IMFs of all star forming regions, should also be invariant. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation we use the IMF determined in resolved star clusters and apply the IGIMF-theory to calculate a grid of gwIMF models for metallicities, -3<[Fe/H]<1, and galaxy-wide star formation rates, $10^{-5}$<SFR<$10^{5}\,\mathrm{M_{\odot}/yr}$. For a galaxy with metallicy [Fe/H]$1\,M_\odot$/yr, which is a common condition in the early Universe, we find that the gwIMF is top-heavy (more massive stars), when compared to the canonical IMF. For a SFR $< 1\,\mathrm{M_{\odot}/yr}$ the gwIMF becomes top-light regardless of the metallicity. For metallicities $\mathrm{[Fe/H]} > 0$ the gwIMF can become bottom-heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top-heavy within the first few hundred Myr of the galaxy's life and that it evolves into a bottom-heavy gwIMF in the metal-enriched galactic center. We study the SFR$-$H$\alpha$ relation, its dependency on metallicity and the SFR, the correction factors to the Kennicutt SFR$_{\rm K}-$H$\alpha$ relation, and provide new fitting functions Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for the gas-consumption time scales and for the main sequence of galaxies. The Leo P and ultra-faint dwarf galaxies are discussed explicitly. [abridged]Comment: Astronomy and Astrophysics (A&A) in press. 15 pages, 8 figure

Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants

Extensive surveys of star-forming regions with Spitzer have revealed populations of disk-bearing young stellar objects. These have provided crucial constraints, such as the timescale of dispersal of protoplanetary disks, obtained by carefully combining infrared data with spectroscopic or X-ray data. While observations in various regions agree with the general trend of decreasing disk fraction with age, the Lupus V and VI regions appeared to have been at odds, having an extremely low disk fraction. Here we show, using the recent Gaia data release 2 (DR2), that these extremely low disk fractions are actually due to a very high contamination by background giants. Out of the 83 candidate young stellar objects (YSOs) in these clouds observed by Gaia, only five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and have similar proper motions to other members in this star-forming complex. Of these five targets, four have optically thick (Class II) disks. On the one hand, this result resolves the conundrum of the puzzling low disk fraction in these clouds, while, on the other hand, it further clarifies the need to confirm the Spitzer selected diskless population with other tracers, especially in regions at low galactic latitude like Lupus V and VI. The use of Gaia astrometry is now an independent and reliable way to further assess the membership of candidate YSOs in these, and potentially other, star-forming regions.Comment: Accepted for publication on Astronomy&Astrophysics Letter

Optical techniques for 3D surface reconstruction in computer-assisted laparoscopic surgery

One of the main challenges for computer-assisted surgery (CAS) is to determine the intra-opera- tive morphology and motion of soft-tissues. This information is prerequisite to the registration of multi-modal patient-specific data for enhancing the surgeon’s navigation capabilites by observ- ing beyond exposed tissue surfaces and for providing intelligent control of robotic-assisted in- struments. In minimally invasive surgery (MIS), optical techniques are an increasingly attractive approach for in vivo 3D reconstruction of the soft-tissue surface geometry. This paper reviews the state-of-the-art methods for optical intra-operative 3D reconstruction in laparoscopic surgery and discusses the technical challenges and future perspectives towards clinical translation. With the recent paradigm shift of surgical practice towards MIS and new developments in 3D opti- cal imaging, this is a timely discussion about technologies that could facilitate complex CAS procedures in dynamic and deformable anatomical regions

Star clusters near and far; tracing star formation across cosmic time

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00690-x.Star clusters are fundamental units of stellar feedback and unique tracers of their host galactic properties. In this review, we will first focus on their constituents, i.e.\ detailed insight into their stellar populations and their surrounding ionised, warm, neutral, and molecular gas. We, then, move beyond the Local Group to review star cluster populations at various evolutionary stages, and in diverse galactic environmental conditions accessible in the local Universe. At high redshift, where conditions for cluster formation and evolution are more extreme, we are only able to observe the integrated light of a handful of objects that we believe will become globular clusters. We therefore discuss how numerical and analytical methods, informed by the observed properties of cluster populations in the local Universe, are used to develop sophisticated simulations potentially capable of disentangling the genetic map of galaxy formation and assembly that is carried by globular cluster populations.Peer reviewedFinal Accepted Versio

Climate, soil organic layer, and nitrogen jointly drive forest development after fire in the North American boreal zone

Previous empirical work has shown that feedbacks between fire severity, soil organic layer thickness, tree recruitment, and forest growth are important factors controlling carbon accumulation after fire disturbance. However, current boreal forest models inadequately simulate this feedback. We address this deficiency by updating the ED2 model to include a dynamic feedback between soil organic layer thickness, tree recruitment, and forest growth. The model is validated against observations spanning monthly to centennial time scales and ranging from Alaska to Quebec. We then quantify differences in forest development after fire disturbance resulting from changes in soil organic layer accumulation, temperature, nitrogen availability, and atmospheric CO2. First, we find that ED2 accurately reproduces observations when a dynamic soil organic layer is included. Second, simulations indicate that the presence of a thick soil organic layer after a mild fire disturbance decreases decomposition and productivity. The combination of the biological and physical effects increases or decreases total ecosystem carbon depending on local conditions. Third, with a 48C temperature increase, some forests transition from undergoing succession to needleleaf forests to recruiting multiple cohorts of broadleaf trees, decreasing total ecosystem carbon by �40% after 300 years. However, the presence of a thick soil organic layer due to a persistently mild fire regime can prevent this transition and mediate carbon losses even under warmer temperatures. Fourth, nitrogen availability regulates successional dynamics; broadleaf species are less competitive with needleleaf trees under low nitrogen regimes. Fifth, the boreal forest shows additional short-term capacity for carbon sequestration as atmospheric CO2 increases