A scaling relation for the molecular cloud lifetime in Milky Way-like galaxies

We study the time evolution of molecular clouds across three Milky Way-like isolated disc galaxy simulations at a temporal resolution of 1 Myr and at a range of spatial resolutions spanning two orders of magnitude in spatial scale from ∼10 pc up to ∼1 kpc. The cloud evolution networks generated at the highest spatial resolution contain a cumulative total of ∼80 000 separate molecular clouds in different galactic-dynamical environments. We find that clouds undergo mergers at a rate proportional to the crossing time between their centroids, but that their physical properties are largely insensitive to these interactions. Below the gas-disc scale height, the cloud lifetime τlife obeys a scaling relation of the form τlife∝ℓ-0.3 with the cloud size ℓ, consistent with over-densities that collapse, form stars, and are dispersed by stellar feedback. Above the disc scale height, these self-gravitating regions are no longer resolved, so the scaling relation flattens to a constant value of ∼13 Myr, consistent with the turbulent crossing time of the gas disc, as observed in nearby disc galaxies.

Stellar clustering shapes the architecture of planetary systems

Planet formation is generally described in terms of a system containing the host star and a protoplanetary disk^1–3, of which the internal properties (for example, mass and metallicity) determine the properties of the resulting planetary system⁴. However, (proto)planetary systems are predicted^5,6 and observed^7,8 to be affected by the spatially clustered stellar formation environment, through either dynamical star–star interactions or external photoevaporation by nearby massive stars⁹. It is challenging to quantify how the architecture of planetary sysems is affected by these environmental processes, because stellar groups spatially disperse within less than a billion years¹⁰, well below the ages of most known exoplanets. Here we identify old, co-moving stellar groups around exoplanet host stars in the astrometric data from the Gaia satellite^11,12 and demonstrate that the architecture of planetary systems exhibits a strong dependence on local stellar clustering in position-velocity phase space. After controlling for host stellar age, mass, metallicity and distance from the star, we obtain highly significant differences (with p values of 10⁻⁵ to 10⁻²) in planetary system properties between phase space overdensities (composed of a greater number of co-moving stars than unstructured space) and the field. The median semi-major axis and orbital period of planets in phase space overdensities are 0.087 astronomical units and 9.6 days, respectively, compared to 0.81 astronomical units and 154 days, respectively, for planets around field stars. ‘Hot Jupiters’ (massive, short-period exoplanets) predominantly exist in stellar phase space overdensities, strongly suggesting that their extreme orbits originate from environmental perturbations rather than internal migration^13,14 or planet–planet scattering^15,16. Our findings reveal that stellar clustering is a key factor setting the architectures of planetary systems.

The Molecular Cloud Lifecycle

Giant molecular clouds (GMCs) and their stellar offspring are the building blocks of galaxies. The physical characteristics of GMCs and their evolution are tightly connected to galaxy evolution. The macroscopic properties of the interstellar medium propagate into the properties of GMCs condensing out of it, with correlations between e.g. the galactic and GMC scale gas pressures, surface densities and volume densities. That way, the galactic environment sets the initial conditions for star formation within GMCs. After the onset of massive star formation, stellar feedback from e.g. photoionisation, stellar winds, and supernovae eventually contributes to dispersing the parent cloud, depositing energy, momentum and metals into the surrounding medium, thereby changing the properties of galaxies. This cycling of matter between gas and stars, governed by star formation and feedback, is therefore a major driver of galaxy evolution. Much of the recent debate has focused on the durations of the various evolutionary phases that constitute this cycle in galaxies, and what these can teach us about the physical mechanisms driving the cycle. We review results from observational, theoretical, and numerical work to build a dynamical picture of the evolutionary lifecycle of GMC evolution, star formation, and feedback in galaxies.

Fast and inefficient star formation due to short-lived molecular clouds and rapid feedback

The physics of star formation and the deposition of mass, momentum, and energy into the interstellar medium by massive stars (‘feedback’) are the main uncertainties in modern cosmological simulations of galaxy formation and evolution1, 2. These processes determine the properties of galaxies3, 4, but are poorly understood on the ≲100 pc scale of individual giant molecular clouds (GMCs)5, 6 resolved in modern galaxy formation simulations7, 8. The key question is why the timescale for depleting molecular gas through star formation in galaxies (t_dep ≈ 2 Gyr)9, 10 exceeds the dynamical timescale of GMCs by two orders of magnitude11. Either most of a GMC’s mass is converted into stars over many dynamical times12, or only a small fraction turns into stars before the GMC is dispersed on a dynamical timescale13, 14. Here we report our observation that molecular gas and star formation are spatially decorrelated on GMC scales in the nearby flocculent spiral galaxy NGC300, contrary to their tight correlation on galactic scales5. We demonstrate that this de-correlation implies rapid evolutionary cycling between GMCs, star formation, and feedback. We apply a novel statistical method15, 16 to quantify the evolutionary timeline and find that star formation is regulated by efficient stellar feedback, driving GMC dispersal on short timescales (~1.5 Myr) due to radiation and stellar winds, prior to supernova explosions. This feedback limits GMC lifetimes to about one dynamical timescale (~10 Myr), with integrated star formation efficiencies of only 2–3%. Our findings reveal that galaxies consist of building blocks undergoing vigorous, feedback-driven lifecycles, that vary with the galactic environment and collectively define how galaxies form stars. Systematic applications of this multi-scale analysis to large galaxy samples will provide key input for a predictive, bottom-up theory of galaxy formation and evolution.

[Anthropology and delusional disorders: Antilles and Cameroon discourse]

We are sometimes compare, in our practice, with treatment of patients coming from beyond the seas (like Guadeloup, West Indies). The psychologic characters are extremely confusing because the symptoms are polymorphs in the same patient, some character of hysteric, delirious and depressive registers, and also there is a cultural color changing the deep signification of symptoms presented to the doctor. The diagnostic research are not unusual face our proper culture and with inadequation of the classic diagnostical characters. The treatment's antilly patients mind the attirance for the exotic and the unknown phobies captivate and question our proper subjectivity.

Study of the unidimensionality of the Bech-Rafaelsen Melancholia Scale using Rasch analysis in a French sample of major depressive disorders

In a French sample of 73 major depressive disorders (according to the DSM III definition), the unidimensionality of the Bech-Rafaelsen Melancholia Scale was studied; firstly by means of traditional but not really valid methods (i.e., factor analysis and Cronbach's alpha), and second, using the Rasch logistic model. These methods would seem to show that the B.R.M.E.S. provides a unidimensional measurement of depression.