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Vegetation can modify the planetary surface albedo via the Charney mechanism, as plants are usually darker than the bare surface of the continents. We updated ESTM (Earth-like Surface Temperature Model) to incorporate the presence, distribution and evolution of two dynamically competing vegetation types that resemble grasslands and trees (the latter in the double stages of life: adults and seedlings). The newly developed model was applied to estimate how the climate-vegetation system reaches equilibrium across different rocky planetary configurations, and to assess its impact on temperature and habitability. With respect to a world with bare granite continents, the effect of vegetation-albedo feedback is to increase the average surface temperature. Since grasses and trees exhibit different albedos, they affect temperature to different degrees. The ultimate impact on climate depends on the outcome of the competition between these vegetation types. The change in albedo due to vegetation extends the habitable zone and enhances the overall planetary habitability beyond its traditional outer edge. This effect is especially relevant for planets that have a larger extension of continents than Earth. For Earth, the semi-major axis d = 1.04 UA represents the turning point where vegetation enhances habitability from h = 0.0 to h = 0.485 (in the grass-dominance case), to h = 0.584 (in the case of coexistence between grasses and trees), and to h = 0.612 (in the tree-dominance case). This illustrates the transition from a snowball state to a planet with intermediate habitability at the outer edge of the circumstellar habitability zone., Comment: 12 pages, 9 figures (17 panels). Accepded for publication on MNRAS on Aug 1st, 2024

Despite decades of scientific research on the subject, the climate of the first 1.5 Gyr of Mars history has not been fully understood yet. Especially challenging is the need to reconcile the presence of liquid water for extended periods of time on the martian surface with the comparatively low insolation received by the planet, a problem which is known as the Faint Young Sun (FYS) Paradox. In this paper we use ESTM, a latitudinal energy balance model with enhanced prescriptions for meridional heat diffusion, and the radiative transfer code EOS to investigate how seasonal variations of temperature can give rise to local conditions which are conductive to liquid water runoffs. We include the effects of the martian dichotomy, a northern ocean with either 150 or 550 m of Global Equivalent Layer (GEL) and simplified CO$_2$ or H$_2$O clouds. We find that 1.3-to-2.0 bar CO$_2$-dominated atmospheres can produce seasonal thaws due to inefficient heat redistribution, provided that the eccentricity and the obliquity of the planet are sufficiently different from zero. We also studied the impact of different values for the argument of perihelion. When local favorable conditions exist, they nearly always persist for $>15\%$ of the martian year. These results are obtained without the need for additional greenhouse gases (e.g. H$_2$, CH$_4$) or transient heat-injecting phenomena (e.g. asteroid impacts, volcanic eruptions). Moderate amounts (0.1 to 1\%) of CH$_4$ significantly widens the parameter space region in which seasonal thaws are possible., Comment: Second and final version, 27 pages, 10 figures, accepted for publication in ApJ

Energy balance models (EBMs) are one- or two-dimensional climate models that can provide insight into planetary atmospheres, particularly with regard to habitability. Because EBMs are far less computationally intensive than three-dimensional general circulation models (GCMs), they can be run over large, uncertain parameter spaces and can be used to explore long-period phenomena like carbon and Milankovitch cycles. Because horizontal dimensions are incorporated in EBMs, they can explore processes that are beyond the reach of one-dimensional radiative-convective models (RCMs). EBMs are, however, dependent on parameterizations and tunings to account for physical processes that are neglected. Thus, EBMs rely on observations and results from GCMs and RCMs. Different EBMs have included a wide range of parameterizations (for albedo, radiation, and heat diffusion) and additional physics, such as carbon cycling and ice sheets. This CUISINES exoplanet model intercomparison project (exoMIP) will compare various EBMs across a set of numerical experiments. The set of experiments will include Earth-like planets at different obliquities, parameter sweeps across obliquity, and variations in instellation and CO$_2$ abundance to produce hysteresis diagrams. We expect a range of different results due to the choices made in the various codes, highlighting which results are robust across models and which are dependent on parameterizations or other modeling choices. Additionally, it will allow developers to identify model defects and determine which parameterizations are most useful or relevant to the problem of interest. Ultimately, this exoMIP will allow us to improve the consistency between EBMs and accelerate the process of discovering habitable exoplanets., Comment: 19 pages, 5 figures, accepted for publication at Planetary Science Journal

We present EOS, a procedure for determining the Outgoing Longwave Radiation (OLR) and top-of-atmosphere (TOA) albedo for a wide range of conditions expected to be present in the atmospheres of rocky planets with temperate conditions. EOS is based on HELIOS and HELIOS-K, which are novel and publicly available atmospheric radiative transfer (RT) codes optimized for fast calculations with GPU processors. These codes were originally developed for the study of giant planets. In this paper we present an adaptation for applications to terrestrial-type, habitable planets, adding specific physical recipes for the gas opacity and vertical structure of the atmosphere. To test the reliability of the procedure we assessed the impact of changing line opacity profile, continuum opacity model, atmospheric lapse rate and tropopause position prescriptions on the OLR and the TOA albedo. The results obtained with EOS are in line with those of other RT codes running on traditional CPU processors, while being at least one order of magnitude faster. The adoption of OLR and TOA albedo data generated with EOS in a zonal and seasonal climate model correctly reproduce the fluxes of the present-day Earth measured by the CERES spacecraft. The results of this study disclose the possibility to incorporate fast RT calculations in climate models aimed at characterizing the atmospheres of habitable exoplanets., Comment: Accepted for publication in ApJ

We study the evolution of Eu and Ba abundances in local group dwarf spheroidal and ultra faint dwarf galaxies by means of detailed chemical evolution models and compare our results with new sets of homogeneous abundances. The adopted models include gas infall and outflow and have been previously tested. We investigate several production scenarios for r-process elements: merging neutron stars and magneto-rotational driven supernovae. Production of Ba through the main s-process acting in low- and intermediate- mass stars is considered as well. We also test different sets of nucleosynthesis yields. For merging neutron stars we adopt either a constant and short delay time for merging or a delay time distribution function. Our simulations show that: i) if r-process elements are produced only by a quick source, it is possible to reproduce the [Eu/Fe] vs [Fe/H], but those models fail in reproducing the [Ba/Fe] vs [Fe/H]. ii) If r-process elements are produced only with longer delays the opposite happens. iii) If both a quick source and a delayed one are adopted, such as magneto-rotational driven supernovae and merging neutron stars with a delay time distribution, the [Eu/Fe] abundance pattern is successfully reproduced, but models still fail in reproducing the [Ba/Fe]. iv) On the other hand, the characteristic abundances of Reticulum II can be reproduced only if both the Eu and the r-process fraction of Ba are produced on short and constant time delays during a single merging event. We discuss also other possible interpretations, including an inhomogeneous mixing of gas which might characterize this galaxy.

Observations of exoplanets and protoplanetary disks show that binary stellar systems can host planets in stable orbits. Given the high binary fraction among stars, the contribution of binary systems to Galactic habitability should be quantified. Therefore, we have designed a suite of Monte Carlo experiments aimed at generating large (up to $10^6$) samples of binary systems. For each system randomly extracted we calculate the intersection between the radiative habitable zones and the regions of dynamical stability using published empirical formulations that account for the dynamical and radiative parameters of both stars of the system. We also consider constraints on planetary formation in binary systems. We find that the habitability properties of circumstellar and circumbinary regions are quite different and complementary with respect to the binary system parameters. Circumbinary HZs are, generally, rare ($\simeq 4\%$) in the global population of binary systems, even if they are common for stellar separations $\lesssim 0.2$ AU. Conversely, circumstellar HZs are frequent ($\ge 80\%$) in the global population, but are rare for stellar separations $\lesssim 1$ AU. These results are robust against variations of poorly constrained binary systems parameters. We derive ranges of stellar separations and stellar masses for which HZs in binary systems can be wider than the HZs around single stars; the widening can be particularly strong (up to one order of magnitude) for circumstellar regions around M-type secondary stars. The comparison of our statistical predictions with observational surveys shows the impact of selection effects on the habitability properties of detected exoplanets in binary systems., Comment: 39 pages and 12 figures. Accepted for publication in ApJ

The merging rate of double neutron stars (DNS) has a great impact on many astrophysical issues, including the interpretation of gravitational waves signals, of the short Gamma Ray Bursts (GRBs), and of the chemical properties of stars in galaxies. Such rate depends on the distribution of the delay times (DDT) of the merging events. In this paper we derive a theoretical DDT of merging DNS following from the characteristics of the clock controlling their evolution. We show that the shape of the DDT is governed by a few key parameters, primarily the lower limit and the slope of the distribution of the separation of the DNS systems at birth. With a parametric approach we investigate on the observational constraints on the DDT from the cosmic rate of short GRBs and the Europium to Iron ratio in Milky Way stars, taken as tracer of the products of the explosion. We find that the local rate of DNS merging requires that about 1 percent of neutron stars progenitors live in binary systems which end their evolution as merging DNS within a Hubble time. The redshift distribution of short GRBs does not yet provide a strong constraint on the shape of the DDT, although the best fitting models have a shallow DDT. The chemical pattern in Milky Way stars requires an additional source of Europium besides the products from merging DNS, which weakens the related requirement on the DDT. At present both constraints can be matched with the same DDT for merging DNS., Comment: 18 pages, to appear in MNRAS

In this work, we compute rates of merging neutron stars (MNS) in galaxies of different morphological type, as well as the cosmic MNS rate in a unitary volume of the Universe adopting different cosmological scenarios. Our aim is to provide predictions of kilonova rates for future observations both at low and high redshift. In the adopted galaxy models, we take into account the production of r-process elements either by MNS or core-collapse supernovae. In computing the MNS rates we adopt either a constant total time delay for merging (10 Myr) or a distribution function of such delays. Our main conclusions are: i) the observed present time MNS rate in our Galaxy is well reproduced either with a constant time delay or a distribution function $\propto t^{-1}$. The [Eu/Fe] vs. [Fe/H] relation in the Milky Way can be well reproduced with only MNS, if the time delay is short and constant. If the distribution function of delays is adopted, core-collapse supernovae as are also required. ii) The present time cosmic MNS rate can be well reproduced in any cosmological scenario, either pure luminosity evolution or a typical hierarchical one, and spirals are the main contributors to it. iii) The spirals are the major contributors to the cosmic MNS at all redshifts in hierarchical scenarios. In the pure luminosity evolution scenario, the spirals are the major contributors locally, whereas at high redshift ellipticals dominate. iv) The predicted cosmic MNS rate well agrees with the cosmic rate of short Gamma Ray Bursts if the distribution function of delays is adopted, in a cosmological hierarchical scenario observationally derived. v) Future observations of Kilonovae in ellipticals will allow to disentangle among constant or a distribution of time delays as well as among different cosmological scenarios.

The merging of two neutron stars (MNS) is thought to be the source of short gamma-ray bursts (SGRB) and gravitational wave transients, as well as the main production site of r-process elements like Eu. We have derived a new delay time distribution (DTD) for MNS systems from theoretical considerations and we have tested it against (i) the SGRB redshift distribution and (ii) the Galactic evolution of Eu and Fe, in particular the [Eu/Fe] vs [Fe/H] relation. For comparison, we also tested other DTDs, as proposed in the literature. To address the first item, we have convolved the DTD with the cosmic star formation rate, while for the second we have employed a detailed chemical evolution model of the Milky Way. We have also varied the DTD of Type Ia SNe (the main Fe producers), the contribution to Eu production from core-collapse SNe, as well as explored the effect of a dependence on the metallicity of the occurrence probability of MNS. Our main results can be summarized as follows: (i) the SGRB redshift distribution can be fitted using DTDs for MNS that produce average timescales of 300-500 Myr; (ii) if the MNS are the sole producers of the Galactic Eu and the occurrence probability of MNS is constant the Eu production timescale must be on the order of <30 Myr; (iii) allowing for the Eu production in core-collapse SNe, or adopting a metallicity-dependent occurrence probability, allow us to reproduce both observational constraints, but many uncertainties are still present in both assumptions., Comment: 17 pages, 14 figures, submitted to MNRAS

Probing the origin of r-process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H]\,$>-1$ in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times ($\propto~t^{-1}$) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions ($\propto~t^{-1.5}$) or power laws combined with a strong burst of mergers before the onset of Type~Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type~Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this extra site could be responsible for roughly 50% of the Eu production in the early universe, before the onset of Type~Ia supernovae. Rare classes of supernovae could be this additional r-process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern., Comment: 29 pages, 10 figures, 1 table, re-submitted to ApJ (revised version)