The Experts below are selected from a list of 207 Experts worldwide ranked by ideXlab platform
Andreas Quirrenbach - One of the best experts on this subject based on the ideXlab platform.
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Precise radial velocities of giant stars. XII. Evidence against the proposed Planet Aldebaran b
Astronomy & Astrophysics, 2019Co-Authors: Katja Reichert, Sabine Reffert, S. Stock, Trifon Trifonov, Andreas QuirrenbachAbstract:Radial-velocity variations of the K giant star Aldebaran ($\alpha$ Tau) were first reported in the early 1990s. After subsequent analyses, the radial-velocity variability with a period of $\sim 629\,\mathrm{d}$ has recently been interpreted as caused by a Planet of several Jovian masses. We want to further investigate the hypothesis of an extrasolar Planet around Aldebaran. We combine 165 new radial-velocity measurements from Lick Observatory with seven already published data sets comprising 373 radial-velocity measurements. We perform statistical analyses and investigate whether a Keplerian model properly fits the radial velocities. We also perform a dynamical stability analysis for a possible two-Planet solution. As best Keplerian fit to the combined radial-velocity data we obtain an orbit for the Hypothetical Planet with a smaller period ($P=607\,\mathrm{d}$) and a larger eccentricity ($e=0.33 \pm 0.04$) than the previously proposed one. However, the residual scatter around that fit is still large, with a standard deviation of $117\,\mathrm{ms}^{-1}$. In 2006/2007, the statistical power of the $\sim 620\,\mathrm{d}$ period showed a temporary but significant decrease. Plotting the growth of power in reverse chronological order reveals that a period around $620\,\mathrm{d}$ is clearly present in the newest data but not in the data taken before $\sim$ 2006. Furthermore, an apparent phase shift between radial-velocity data and orbital solution is observable at certain times. A two-Planet Keplerian fit matches the data considerably better than a single-Planet solution, but poses severe dynamical stability issues. The radial-velocity data from Lick Observatory do not further support but in fact weaken the hypothesis of a substellar companion around Aldebaran. Oscillatory convective modes might be a plausible alternative explanation of the observed radial-velocity variations.
Adriana Valio - One of the best experts on this subject based on the ideXlab platform.
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Surface and oceanic habitability of Trappist-1 Planets under the impact of flares
Astrobiology, 2020Co-Authors: Raissa Estrela, Sourav Palit, Adriana ValioAbstract:The discovery of potentially habitable Planets around the ultracool dwarf star Trappist-1 naturally poses the question: could Trappist-1 Planets be home to life? These Planets orbit very close to the host star and are most susceptible to the UV radiation emitted by the intense and frequent flares of Trappist-1. Here we calculate the UV spectra (100 - 450 nm) of a superflare observed on Trappist-1 with the K2 mission. We couple radiative transfer models to this spectra to estimate the UV surface flux on Planets in the habitable zone of Trappist-1 (Planets $e$, $f$, and $g$), assuming atmospheric scenarios based on a pre-biotic and an oxygenic atmosphere. We quantify the impact of the UV radiation on living organisms on the surface and on a Hypothetical Planet ocean. Finally, we find that for non-oxygenic Planets, UV resistant lifeforms would survive on the surface of Planets f and g. Nevertheless, more fragile organisms (i.e. \textit{E. coli}) could be protected from the hazardous UV effects at ocean depths greater than 8m. If the Planets have an ozone layer, any lifeforms studied here would survive in the HZ Planets.
Lorenzo Iorio - One of the best experts on this subject based on the ideXlab platform.
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What would happen if we were about 1 pc away from a supermassive black hole
The Astrophysical Journal, 2020Co-Authors: Lorenzo IorioAbstract:We consider a Hypothetical Planet with the same mass $m$, radius $R$, angular momentum $\boldsymbol{S}$, oblateness $J_2$, semimajor axis $a$, eccentricity $e$, inclination $I$, and obliquity $\varepsilon$ of the Earth orbiting a main-sequence star with the same mass $M_\star$ and radius $R_\star$ of the Sun at a distance $r_\bullet \simeq 1\,\mathrm{parsec}\,\left(\mathrm{pc}\right)$ from a supermassive black hole in the center of the hosting galaxy with the same mass $M_\bullet$ of, say, $\mathrm{M87}^\ast$. We preliminarily investigate some dynamical consequences of its presence in the neighborhood of such a stellar system on the Planet's possibility of sustaining complex life over time. In particular, we obtain general analytic expressions for the long-term rates of change, doubly averaged over both the Planetary and the galactocentric orbital periods $P_\mathrm{b}$ and $P_\bullet$, of $e,\,I,\,\varepsilon$, which are the main quantities directly linked to the stellar insolation. We find that, for certain orbital configurations, the Planet's perihelion distance $q=a\left(1-e\right)$ may greatly shrink and lead to, in some cases, an impact with the star. $I$ may also notably change, with variations even of the order of tens of degrees. On the other hand, $\varepsilon$ does not seem to be particularly affected, being shifted, at most, by $\simeq 0^\circ.02$ over 1 Myr. Our results strongly depend on the eccentricity $e_\bullet$ of the galactocentric motion.
Katja Reichert - One of the best experts on this subject based on the ideXlab platform.
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Precise radial velocities of giant stars. XII. Evidence against the proposed Planet Aldebaran b
Astronomy & Astrophysics, 2019Co-Authors: Katja Reichert, Sabine Reffert, S. Stock, Trifon Trifonov, Andreas QuirrenbachAbstract:Radial-velocity variations of the K giant star Aldebaran ($\alpha$ Tau) were first reported in the early 1990s. After subsequent analyses, the radial-velocity variability with a period of $\sim 629\,\mathrm{d}$ has recently been interpreted as caused by a Planet of several Jovian masses. We want to further investigate the hypothesis of an extrasolar Planet around Aldebaran. We combine 165 new radial-velocity measurements from Lick Observatory with seven already published data sets comprising 373 radial-velocity measurements. We perform statistical analyses and investigate whether a Keplerian model properly fits the radial velocities. We also perform a dynamical stability analysis for a possible two-Planet solution. As best Keplerian fit to the combined radial-velocity data we obtain an orbit for the Hypothetical Planet with a smaller period ($P=607\,\mathrm{d}$) and a larger eccentricity ($e=0.33 \pm 0.04$) than the previously proposed one. However, the residual scatter around that fit is still large, with a standard deviation of $117\,\mathrm{ms}^{-1}$. In 2006/2007, the statistical power of the $\sim 620\,\mathrm{d}$ period showed a temporary but significant decrease. Plotting the growth of power in reverse chronological order reveals that a period around $620\,\mathrm{d}$ is clearly present in the newest data but not in the data taken before $\sim$ 2006. Furthermore, an apparent phase shift between radial-velocity data and orbital solution is observable at certain times. A two-Planet Keplerian fit matches the data considerably better than a single-Planet solution, but poses severe dynamical stability issues. The radial-velocity data from Lick Observatory do not further support but in fact weaken the hypothesis of a substellar companion around Aldebaran. Oscillatory convective modes might be a plausible alternative explanation of the observed radial-velocity variations.
Herve Beust - One of the best experts on this subject based on the ideXlab platform.
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Orbital clustering of distant Kuiper belt objects by Hypothetical Planet 9. Secular or resonant
Astronomy & Astrophysics, 2016Co-Authors: Herve BeustAbstract:Context. Statistical analysis of the orbits of distant Kuiper belt objects (KBOs) has led to the suggestion that an additional Planet should reside in the solar system. According to recent models, the secular action of this body should cause orbital alignment of the KBOs. Aims. It was recently claimed that the KBOs affected by these dynamics are presumably trapped in mean-motion resonances with the suspected Planet. I reinvestigate here the secular model underlying this idea. Methods. The original analysis was carried out by expanding and truncating the secular Hamiltonian. I show that this is inappropriate, as the series expansion is not convergent. I present a study based on numerical computation of the Hamiltonian with no expansion. Results. I show in phase-space diagrams the existence of apsidally anti-aligned, high eccentricity libration islands that were not present in the original modelling, but match numerical simulations. These island were claimed to correspond to bodies trapped in mean-motion resonances with the Hypothetical Planet and match the characteristics of the distant KBOs observed. Conclusions. My main result is that regular secular dynamics can account for the anti-aligned particles itself as well as mean-motion resonances. I also perform a semi-analytical study of resonant motion and show that some resonance are actually capable of producing the same libration islands. I then discuss the relative importance of both mechanisms.
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orbital clustering of distant kuiper belt objects by Hypothetical Planet 9 secular or resonant
arXiv: Earth and Planetary Astrophysics, 2016Co-Authors: Herve BeustAbstract:Statistical analysis of the orbits of distant Kuiper Belt Objects (KBOs) have led to suggest that an additional Planet should reside in the Solar System. According to recent models, the secular action of this body should cause orbital alignment of the KBOs. It was recently claimed that the KBOs concerned by this dynamics are presumably trapped in mean motion resonances with the suspected Planet. I reinvestigate here the secular model underlying this idea. The original analysis was done expanding and truncating the secular Hamiltonian. I show that this is inappropriate here, as the series expansion is not convergent. I present a study based on numerical computation of the Hamiltonian with no expansion. I show in phase-space diagrams the existence of apsidally anti-aligned, high eccentricity libration islands that were not present in the original modelling, but that match numerical simulations. These island were claimed to correspond to bodies trapped in mean-motion resonances with the Hypothetical Planet, and match the characteristics of the distant KBOs observed. My main result is that regular secular dynamics can account for the anti-aligned particles itself as well as mean-motion resonances. I also perform a semi-analytical study of resonant motion and show that some resonance are actually capable of producing the same libration islands. I discuss then the relative importance of both mechanisms.