International Space Science Institute

About: International Space Science Institute is a nonprofit organization based out in Bern, Switzerland. It is known for research contribution in the topics: Solar wind & Comet. The organization has 120 authors who have published 858 publications receiving 25772 citations. The organization is also known as: ISSI.

From Disks to Planets: The Making of Planets and Their Early Atmospheres. An Introduction

Abstract: This paper is an introduction to volume 56 of the Space Science Series of ISSI, “From disks to planets—the making of planets and their proto-atmospheres”, a key subject in our quest for the origins and evolutionary paths of planets, and for the causes of their diversity. Indeed, as exoplanet discoveries progressively accumulated and their characterization made spectacular progress, it became evident that the diversity of observed exoplanets can in no way be reduced to the two classes of planets that we are used to identify in the solar system, namely terrestrial planets and gas or ice giants: the exoplanet reality is just much broader. This fact is no doubt the result of the exceptional diversity of the evolutionary paths linking planetary systems as a whole as well as individual exoplanets and their proto-atmospheres to their parent circumstellar disks: this diversity and its causes are exactly what this paper explores. For each of the main phases of the formation and evolution of planetary systems and of individual planets, we summarize what we believe we understand and what are the important open questions needing further in-depth examination, and offer some suggestions on ways towards solutions. We start with the formation mechanisms of circumstellar disks, with their gas and disk components in which chemical composition plays a very important role in planet formation. We summarize how dust accretion within the disk generates planet cores, while gas accretion on these cores can lead to the diversity of their fluid envelopes. The temporal evolution of the parent disk itself, and its final dissipation, put strong constraints on how and how far planetary formation can proceed. The radiation output of the central star also plays an important role in this whole story. This early phase of planet evolution, from disk formation to dissipation, is characterized by a co-evolution of the disk and its daughter planets. During this co-evolution, planets and their protoatmospheres not only grow, but they also migrate radially as a result of their interaction with the disk, thus moving progressively from their distance of formation to their final location. The formation of planetary fluid envelopes (proto-atmospheres and oceans), is an essential product of this planet formation scenario which strongly constrains their possible evolution towards habitability. We discuss the effects of the initial conditions in the disk, of the location, size and mass of the planetary core, of the disk lifetime and of the radiation output and activity of the central star, on the formation of these envelopes and on their relative extensions with respect to the planet core. Overall, a fraction of the planets retain the primary proto-atmosphere they initially accreted from the gas disk. For those which lose it in this early evolution, outgassing of volatiles from the planetary core and mantle, together with some contributions of volatiles from colliding bodies, give them a chance to form a “secondary” atmosphere, like that of our own Earth. When the disk finally dissipates, usually before 10 Million years of age, it leaves us with the combination of a planetary system and a debris disk, each with a specific radial distribution with respect to their parent star(s). Whereas the dynamics of protoplanetary disks is dominated by gas-solid dynamical coupling, debris disks are dominated by gravitational dynamics acting on diverse families of planetesimals. Solid-body collisions between them and giant impacts on young planetary surfaces generate a new population of gas and dust in those disks. Synergies between solar system and exoplanet studies are particularly fruitful and need to be stimulated even more, because they give access to different and complementary components of debris disks: whereas the different families of planetesimals can be extensively studied in the solar system, they remain unobserved in exoplanet systems. But, in those systems, long-wavelength telescopic observations of dust provide a wealth of indirect information about the unobserved population of planetesimals. Promising progress is being currently made to observe the gas component as well, using millimetre and sub-millimetre giant radio interferometers. Within planetary systems themselves, individual planets are the assembly of a solid body and a fluid envelope, including their planetary atmosphere when there is one. Their characteristics range from terrestrial planets through sub-Neptunes and Neptunes and to gas giants, each type covering most of the orbital distances probed by present-day techniques. With the continuous progress in detection and characterization techniques and the advent of major providers of new data like the Kepler mission, the architecture of these planetary systems can be studied more and more accurately in a statistically meaningful sense and compared to the one of our own solar system, which does not appear to be an exceptional case. Finally, our understanding of exoplanets atmospheres has made spectacular advances recently using the occultation spectroscopy techniques implemented on the currently operating space and ground-based observing facilities. The powerful new observing facilities planned for the near and more distant future will make it possible to address many of the most challenging current questions of the science of exoplanets and their systems. There is little doubt that, using this new generation of facilities, we will be able to reconstruct more and more accurately the complex evolutionary paths which link stellar genesis to the possible emergence of habitable worlds.

First Observational Signature of Rotational Deceleration in a Massive, Intermediate-age Star Cluster in the Magellanic Clouds

Abstract: While the extended main-sequence turn-offs (eMSTOs) found in almost all 1--2 Gyr-old star clusters in the Magellanic Clouds are often explained by postulating extended star-formation histories, the tight subgiant branches (SGBs) seen in some clusters challenge this popular scenario. Puzzlingly, the SGB of the eMSTO cluster NGC 419 is significantly broader at bluer than at redder colors. We carefully assess and confirm the reality of this observational trend. If we would assume that the widths of the features in color--magnitude space were entirely owing to a range in stellar ages, the star-formation histories of the eMSTO stars and the blue SGB region would be significantly more prolonged than that of the red part of the SGB. This cannot be explained by assuming an internal age spread. We show that rotational deceleration of a population of rapidly rotating stars, a currently hotly debated alternative scenario, naturally explains the observed trend along the SGB. Our analysis shows that a `converging' SGB could be produced if the cluster is mostly composed of rapidly rotating stars that slow down over time owing to the conservation of angular momentum during their evolutionary expansion from main-sequence turn-off stars to red giants.

New insights into the formation of the blue main sequence in NGC 1850

Abstract: Recent discoveries of bimodal main sequences (MSs) associated with young clusters (with ages $\lesssim 1$ Gyr) in the Magellanic Clouds have drawn a lot of attention. One of the prevailing formation scenarios attributes these split MSs to a bimodal distribution in stellar rotation rates, with most stars belonging to a rapidly rotating population. In this scenario, only a small fraction of stars populating a secondary blue sequence are slowly or non-rotating stars. Here, we focus on the blue MS in the young cluster NGC 1850. We compare the cumulative number fraction of the observed blue-MS stars to that of the high-mass-ratio binary systems at different radii. The cumulative distributions of both populations exhibit a clear anti-correlation, characterized by a highly significant Pearson coefficient of $-0.97$. Our observations are consistent with the possibility that blue-MS stars are low-mass-ratio binaries, and therefore their dynamical disruption is still ongoing. High-mass-ratio binaries, on the other hand, are more centrally concentrated.

OGLE-2014-BLG-1186: gravitational microlensing providing evidence for a planet orbiting the foreground star or for a close binary source?

Abstract: Discussing the particularly long gravitational microlensing event OGLE-2014-BLG-1186 with a time-scale tE ∼ 300 d, we present a methodology for identifying the nature of localised deviations from single-lens point-source light curves, which ensures that 1) the claimed signal is substantially above the noise floor, 2) the inferred properties are robustly determined and their estimation is not subject to confusion with systematic noise in the photometry, 3) alternative viable solutions within the model framework are not missed. Annual parallax and binarity could be separated and robustly measured from the wing and the peak data, respectively. We find matching model light curves that involve either a binary lens or a binary source, and discover hitherto unknown model ambiguities. Our binary-lens models indicate a planet of mass M2 = (45 ± 9) M⊕, orbiting a star of mass M1 = (0.35 ± 0.06) M⊙, located at a distance DL = (1.7 ± 0.3) kpc from Earth, whereas our binary-source models suggest a brown-dwarf lens of M = (0.046 ± 0.007) M⊙, located at a distance DL = (5.7 ± 0.9) kpc, with the source potentially being a (partially) eclipsing binary involving stars predicted to be of similar colour given the ratios between the luminosities and radii. Further observations might resolve the ambiguity in the interpretation in favour of either a lens or a source binary. We experienced that close binary source stars pose a challenge for claiming the detection of planets by microlensing in events where the source passes very close to the lens star hosting the planet.

Characterization of new hard X-ray Cataclysmic Variables

Abstract: We aim at characterizing a sample of 9 new hard X-ray selected Cataclysmic Variable (CVs), to unambiguously identify them as magnetic systems of the Intermediate Polar (IP) type. We performed timing and spectral analysis by using X-ray, and simultaneous UV and optical data collected by XMM-Newton, complemented with hard X-ray data provided by INTEGRAL and Swift. The pulse arrival time were used to estimate the orbital periods. The X-ray spectra were fitted using composite models consisting of different absorbing columns and emission components. Strong X-ray pulses at the White Dwarf (WD) spin period are detected and found to decrease with energy. Most sources are spin-dominated systems in the X-rays, though four are beat dominated at optical wavelengths. We estimated the orbital period in all system (except for IGR J16500-3307), providing the first estimate for IGR J08390-4833, IGR J18308-1232, and IGR J18173-2509. All X-ray spectra are multi-temperature. V2069 Cyg and RX J0636+3535 posses a soft X-ray optically thick component at kT 80 eV. An intense K_alpha Fe line at 6.4 keV is detected in all sources. An absorption edge at 0.76 keV from OVII is detected in IGR J08390-4833. The WD masses and lower limits to the accretion rates are estimated. We found all sources to be IPs. IGR J08390-4833, V2069 Cyg, and IGR J16500-3307 are pure disc accretors, while IGR J18308-1232, IGR J1509-6649, IGR J17195-4100, and RX J0636+3535 display a disc-overflow accretion mode. All sources show a temperature gradient in the post-shock regions and a highly absorbed emission from material located in the pre-shock flow which is also responsible for the X-ray pulsations. Reflection at the WD surface is likely the origin of the fluorescent iron line. There is an increasing evidence for the presence of a warm absorber in IPs. The addition of 2 systems to the subgroup of soft X-ray IPs confirms a \sim 30% incidence.