472 research outputs found
Feedback in Clouds II: UV Photoionisation and the first supernova in a massive cloud
Molecular cloud structure is regulated by stellar feedback in various forms.
Two of the most important feedback processes are UV photoionisation and
supernovae from massive stars. However, the precise response of the cloud to
these processes, and the interaction between them, remains an open question. In
particular, we wish to know under which conditions the cloud can be dispersed
by feedback, which in turn can give us hints as to how feedback regulates the
star formation inside the cloud. We perform a suite of radiative
magnetohydrodynamic simulations of a 10^5 solar mass cloud with embedded
sources of ionising radiation and supernovae, including multiple supernovae and
a hypernova model. A UV source corresponding to 10% of the mass of the cloud is
required to disperse the cloud, suggesting that the star formation efficiency
should be on the order of 10%. A single supernova is unable to significantly
affect the evolution of the cloud. However, energetic hypernovae and multiple
supernovae are able to add significant quantities of momentum to the cloud,
approximately 10^{43} g cm/s of momentum per 10^{51} ergs of supernova energy.
This is on the lower range of estimates in other works, since dense gas clumps
that remain embedded inside the HII region cause rapid cooling in the supernova
blast. We argue that supernovae alone are unable to regulate star formation in
molecular clouds, and that strong pre-supernova feedback is required to allow
supernova blastwaves to propagate efficiently into the interstellar mediumComment: 15 pages, 10 figures, submitted to MNRA
3D simulations of pillars formation around HII regions: the importance of shock curvature
Radiative feedback from massive stars is a key process to understand how HII
regions may enhance or inhibit star formation in pillars and globules at the
interface with molecular clouds. We aim to contribute to model the interactions
between ionization and gas clouds to better understand the processes at work.
We study in detail the impact of modulations on the cloud-HII region interface
and density modulations inside the cloud. We run three-dimensional
hydrodynamical simulations based on Euler equations coupled with gravity using
the HERACLES code. We implement a method to solve ionization/recombination
equations and we take into account typical heating and cooling processes at
work in the interstellar medium and due to ionization/recombination physics. UV
radiation creates a dense shell compressed between an ionization front and a
shock ahead. Interface modulations produce a curved shock that collapses on
itself leading to stable growing pillar-like structures. The narrower the
initial interface modulation, the longer the resulting pillar. We interpret
pillars resulting from density modulations in terms of the ability of these
density modula- tions to curve the shock ahead the ionization front. The shock
curvature is a key process to understand the formation of structures at the
edge of HII regions. Interface and density modulations at the edge of the cloud
have a direct impact on the morphology of the dense shell during its formation.
Deeper in the cloud, structures have less influence due to the high densities
reached by the shell during its expansion.Comment: Accepted by A&A 03/11/201
The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres
The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest
known brown dwarfs, with effective temperatures T_eff <= 475 K. The first
published observations provide important information: not only is the
atmospheric chemistry out of equilibrium, as previously known, but the
pressure-temperature profile is not in the standard adiabatic form. The rapid
rotation of these Jupiter-size, isolated, brown dwarfs dominates the
atmospheric dynamics, and thermal and compositional changes disrupt convection.
These processes produce a colder lower atmosphere, and a warmer upper
atmosphere, compared to a standard adiabatic profile. Leggett et al. (2021)
presented empirical models where the pressure-temperature profile was adjusted
so that synthetic spectra reproduced the 1 <= lambda um <= 20 spectral energy
distributions of brown dwarfs with 260 <= T_eff K <= 540. We show that spectra
generated by these models fit the first JWST Y dwarf spectrum better than
standard-adiabat models. Unexpectedly, there is no 4.3 um PH_3 feature in the
JWST spectrum and atmospheres without phosphorus better reproduce the 4 um flux
peak. Our analysis of new JWST photometry indicates that the recently
discovered faint secondary of the WISE J033605.05-014350AB system
(Calissendorff et al. 2023) has T_eff = 295 K, making it the first dwarf in the
significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all
other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are
recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is
publicly available. Photometric color transformations are provided in an
Appendix.Comment: Accepted for publication in ApJ on 25 September 202
Fingering convection and cloudless models for cool brown dwarf atmospheres
This work aims to improve the current understanding of the atmospheres of
brown dwarfs, especially cold ones with spectral type T and Y, whose modeling
is a current challenge. Silicate and iron clouds are believed to disappear at
the photosphere at the L/T transition, but cloudless models fail to reproduce
correctly the spectra of T dwarfs, advocating for the addition of more physics,
e.g. other types of clouds or internal energy transport mechanisms. We use a
one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate
this issue. This code includes both equilibrium and out-of-equilibrium
chemistry and solves consistently the PT structure. Included opacity sources
are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are
present in the atmosphere. We show that the spectra of Y dwarfs can be
accurately reproduced with a cloudless model if vertical mixing and NH3
quenching are taken into account. T dwarf spectra still have some reddening in
e.g. J - H compared to cloudless models. This reddening can be reproduced by
slightly reducing the temperature gradient in the atmosphere. We propose that
this reduction of the stabilizing temperature gradient in these layers, leading
to cooler structures, is due to the onset of fingering convection, triggered by
the destabilizing impact of condensation of very thin dust.Comment: Accepted in ApJ
A uniform analysis of HD209458b Spitzer/IRAC lightcurves with Gaussian process models
We present an analysis of Spitzer/IRAC primary transit and secondary eclipse
lightcurves measured for HD209458b, using Gaussian process models to
marginalise over the intrapixel sensitivity variations in the 3.6 micron and
4.5 micron channels and the ramp effect in the 5.8 micron and 8.0 micron
channels. The main advantage of this approach is that we can account for a
broad range of degeneracies between the planet signal and systematics without
actually having to specify a deterministic functional form for the latter. Our
results do not confirm a previous claim of water absorption in transmission.
Instead, our results are more consistent with a featureless transmission
spectrum, possibly due to a cloud deck obscuring molecular absorption bands.
For the emission data, our values are not consistent with the thermal inversion
in the dayside atmosphere that was originally inferred from these data.
Instead, we agree with another re-analysis of these same data, which concluded
a non-inverted atmosphere provides a better fit. We find that a solar-abundance
clear-atmosphere model without a thermal inversion underpredicts the measured
emission in the 4.5 micron channel, which may suggest the atmosphere is
depleted in carbon monoxide. An acceptable fit to the emission data can be
achieved by assuming that the planet radiates as an isothermal blackbody with a
temperature of K.Comment: 18 pages, 5 figures, 6 tables. Accepted by MNRA
Understanding star formation in molecular clouds I. Effects of line-of-sight contamination on the column density structure
Column-density maps of molecular clouds are one of the most important
observables in the context of molecular cloud- and star-formation (SF) studies.
With the Herschel satellite it is now possible to determine the column density
from dust emission. We use observations and simulations to demonstrate how LOS
contamination affects the column density probability distribution function
(PDF). We apply a first-order approximation (removing a constant level) to the
molecular clouds of Auriga, Maddalena, Carina and NGC3603. In perfect agreement
with the simulations, we find that the PDFs become broader, the peak shifts to
lower column densities, and the power-law tail of the PDF flattens after
correction. All PDFs have a lognormal part for low column densities with a peak
at Av~2, a deviation point (DP) from the lognormal at Av(DP)~4-5, and a
power-law tail for higher column densities. Assuming a density distribution
rho~r^-alpha, the slopes of the power-law tails correspond to alpha(PDF)=1.8,
1.75, and 2.5 for Auriga, Carina, and NGC3603 (alpha~1.5-2 is consistent
gravitational collapse). We find that low-mass and high-mass SF clouds display
differences in the overall column density structure. Massive clouds assemble
more gas in smaller cloud volumes than low-mass SF ones. However, for both
cloud types, the transition of the PDF from lognormal shape into power-law tail
is found at the same column density (at Av~4-5 mag). Low-mass and high-mass SF
clouds then have the same low column density distribution, most likely
dominated by supersonic turbulence. At higher column densities, collapse and
external pressure can form the power-law tail. The relative importance of the
two processes can vary between clouds and thus lead to the observed differences
in PDF and column density structure.Comment: A&A accepted, 15.12. 201
Near-Infrared Spectroscopy of the Y0 WISEP J173835.52+273258.9 and the Y1 WISE J035000.32-565830.2: the Importance of Non-Equilibrium Chemistry
We present new near-infrared spectra, obtained at Gemini Observatory, for two
Y dwarfs: WISE J035000.32-565830.2 (W0350) and WISEP J173835.52+273258.9
(W1738). A FLAMINGOS-2 R=540 spectrum was obtained for W0350, covering 1.0 <
lambda um < 1.7, and a cross-dispersed GNIRS R=2800 spectrum was obtained for
W1738, covering 0.993-1.087 um, 1.191-1.305 um, 1.589-1.631 um, and 1.985-2.175
um, in four orders. We also present revised YJH photometry for W1738, using new
NIRI Y and J imaging, and a re-analysis of the previously published NIRI H band
images. We compare these data, together with previously published data for
late-T and Y dwarfs, to cloud-free models of solar metallicity, calculated both
in chemical equilibrium and with disequilibrium driven by vertical transport.
We find that for the Y dwarfs the non-equilibrium models reproduce the
near-infrared data better than the equilibrium models. The remaining
discrepancies suggest that fine-tuning the CH_4/CO and NH_3/N_2 balance is
needed. Improved trigonometric parallaxes would improve the analysis. Despite
the uncertainties and discrepancies, the models reproduce the observed
near-infrared spectra well. We find that for the Y0, W1738, T_eff = 425 +/- 25
K and log g = 4.0 +/- 0.25, and for the Y1, W0350, T_eff = 350 +/- 25 K and log
g = 4.0 +/- 0.25. W1738 may be metal-rich. Based on evolutionary models, these
temperatures and gravities correspond to a mass range for both Y dwarfs of 3-9
Jupiter masses, with W0350 being a cooler, slightly older, version of W1738;
the age of W0350 is 0.3-3 Gyr, and the age of W1738 is 0.15-1 Gyr.Comment: Accepted on March 30 2016 for publication in Ap
Ionization Compression Impact On Dense Gas Distribution And Star Formation - Probability Density Functions Around H Ii Regions As Seen By Herschel
Aims. Ionization feedback should impact the probability distribution function (PDF) of the column density of cold dust around the ionized gas. We aim to quantify this effect and discuss its potential link to the core and initial mass function (CMF/IMF)
Pillars And Globules At The Edges Of H Ii Regions: Confronting Herschel Observations And Numerical Simulations
Herschel far-infrared imaging observations have revealed the density structure of the interface between H ii regions and molecular clouds in great detail. In particular, pillars and globules are present in many high-mass star-forming regions, such as the Eagle nebula (M 16) and the Rosette molecular cloud, and understanding their origin will help characterize triggered star formation
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