اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدHow Common are Extrasolar, Late Heavy Bombardments?
258
0
0.0
(
0
)
تأليف
Mark Booth
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The habitability of planets is strongly affected by impacts from comets and asteroids. Indications from the ages of Moon rocks suggest that the inner Solar System experienced an increased rate of impacts roughly 3.8 Gya known as the Late Heavy Bombardment (LHB). Here we develop a model of how the Solar System would have appeared to a distant observer during its history based on the Nice model of Gomes et al. (2005). We compare our results with observed debris discs. We show that the Solar System would have been amongst the brightest of these systems before the LHB. Comparison with the statistics of debris disc evolution shows that such heavy bombardment events must be rare occurring around less than 12% of Sun-like stars.
قيم البحث
اقرأ أيضاً
We introduce a probabilistic approach to the problem of counting dwarf satellites around host galaxies in databases with limited redshift information. This technique is used to investigate the occurrence of satellites with luminosities similar to the
Magellanic Clouds around hosts with properties similar to the Milky Way in the object catalog of the Sloan Digital Sky Survey. Our analysis uses data from SDSS Data Release 7, selecting candidate Milky-Way-like hosts from the spectroscopic catalog and candidate analogs of the Magellanic Clouds from the photometric catalog. Our principal result is the probability for a Milky-Way-like galaxy to host N_{sat} close satellites with luminosities similar to the Magellanic Clouds. We find that 81 percent of galaxies like the Milky Way are have no such satellites within a radius of 150 kpc, 11 percent have one, and only 3.5 percent of hosts have two. The probabilities are robust to changes in host and satellite selection criteria, background-estimation technique, and survey depth. These results demonstrate that the Milky Way has significantly more satellites than a typical galaxy of its luminosity; this fact is useful for understanding the larger cosmological context of our home galaxy.
We utilize observations of 16 white dwarf stars to calculate and analyze the oxidation states of the parent bodies accreting onto the stars. Oxygen fugacity, a measure of overall oxidation state for rocks, is as important as pressure and temperature
in determining the structure of a planet. We find that most of the extrasolar rocky bodies formed under oxidizing conditions, but approximately 1/4 of the polluted white dwarfs have compositions consistent with more reduced parent bodies. The difficulty in constraining the oxidation states of relatively reduced bodies is discussed and a model for the time-dependent evolution of the apparent oxygen fugacity for a hypothetical reduced body engulfed by a WD is investigated. Differences in diffusive fluxes of various elements through the WD envelope yield spurious inferred bulk elemental compositions and oxidation states of the accreting parent bodies under certain conditions. The worst case for biasing against detection of reduced bodies occurs for high effective temperatures. For moderate and low effective temperatures, evidence for relatively reduced parent bodies is preserved under most circumstances for at least several characteristic lifetimes of the debris disk.
The observed Galactic rate of stellar mergers or the initiation of common envelope phases brighter than M_V=-3 (M_I=-4) is of order 0.5 (0.3)/year with 90% confidence statistical uncertainties of 0.24-1.1 (0.14-0.65) and factor of 2 systematic uncert
ainties. The (peak) luminosity function is roughly dN/dL L^(-1.4+/-0.3), so the rates for events more luminous than V1309 Sco (M_V=-7 mag) or V838Mon (M_V=-10 mag) are lower at r~0.1/year and 0.03/year, respectively. The peak luminosity is a steep function of progenitor mass, L M^(2-3). This very roughly parallels the scaling of luminosity with mass on the main sequence, but the transients are ~2000-4000 times more luminous at peak. Combining these, the mass function of the progenitors, dN/dM M^(-2.0+/-0.8), is consistent with the initial mass function, albeit with broad uncertainties. These observational results are also broadly consistent with the estimates of binary population synthesis models. While extragalactic variability surveys can better define the rates and properties of the high luminosity events, systematic, moderate depth (I>16 mag) surveys of the Galactic plane are needed to characterize the low luminosity events. The existing Galactic samples are only ~20% complete and Galactic surveys are (at best) reaching a typical magnitude limit of <13 mag.
The two closest Gamma-Ray Bursts so far detected (GRBs 980425 & 060218) were both under-luminous, spectrally soft, long duration bursts with smooth, single-peaked light curves. Only of the order of 100 GRBs have measured redshifts, and there are, for
example, 2704 GRBs in the BATSE catalogue alone. It is therefore plausible that other nearby GRBs have been observed but not identified as relatively nearby. Here we search for statistical correlations between BATSE long duration GRBs and galaxy samples with recession velocities v <= 11,000 km/s (z = 0.0367, ~ 155 Mpc) selected from two catalogues of nearby galaxies. We also examine the correlations using burst sub-samples restricted to those with properties similar to the two known nearby bursts. Our results show correlation of the entire long GRB sample to remain consistent with zero out to the highest radii considered whereas a sub-sample selected to be low fluence, spectrally soft, with smooth single-peaked light curves (177 bursts) demonstrates increased correlation with galaxies within ~ 155 Mpc. The measured correlation (28% +/- 16% of the sample) suggests that BATSE observed between 2 and 9 long duration GRBs per year similar to, and from within similar distances to GRBs 980425 and 060218. This implies an observed local rate density (to BATSE limits) of 700 +/- 360 Gpc^{-3}yr^{-1} within 155 Mpc.
The dust production in debris discs by grinding collisions of planetesimals requires their orbits to be stirred. However, stirring levels remain largely unconstrained, and consequently the stirring mechanisms as well. This work shows how the sharpnes
s of the outer edge of discs can be used to constrain the stirring levels. Namely, the sharper the edge is the lower the eccentricity dispersion must be. For a Rayleigh distribution of eccentricities ($e$), I find that the disc surface density near the outer edge can be parametrised as $tanh[(r_{max}-r)/l_{rm out}]$, where $r_{max}$ approximates the maximum semi-major axis and $l_{rm out}$ defines the edge smoothness. If the semi-major axis distribution has sharp edges $e_mathrm{rms}$ is roughly $1.2 l_{rm out}/r_{max}$, or $e_mathrm{rms}=0.77 l_{rm out}/r_{max}$ if semi-major axes have diffused due to self-stirring. This model is fitted to ALMA data of five wide discs: HD 107146, HD 92945, HD 206893, AU Mic and HR 8799. The results show that HD 107146, HD 92945 and AU Mic have the sharpest outer edges, corresponding to $e_mathrm{rms}$ values of $0.121pm0.05$, $0.15^{+0.07}_{-0.05}$ and $0.10pm0.02$ if their discs are self-stirred, suggesting the presence of Pluto-sized objects embedded in the disc. Although these stirring values are larger than typically assumed, the radial stirring of HD 92945 is in good agreement with its vertical stirring constrained by the disc height. HD 206893 and HR~8799, on the other hand, have smooth outer edges that are indicative of scattered discs since both systems have massive inner companions.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
