134 research outputs found
MaGICC baryon cycle: the enrichment history of simulated disc galaxies
Using cosmological galaxy formation simulations from the MaGICC (Making Galaxies in a Cosmological Context) project, spanning stellar mass from ∼107 to 3 × 1010 M⊙, we trace the baryonic cycle of infalling gas from the virial radius through to its eventual participation in the star formation process. An emphasis is placed upon the temporal history of chemical enrichment during its passage through the corona and circumgalactic medium. We derive the distributions of time between gas crossing the virial radius and being accreted to the star-forming region (which allows for mixing within the corona), as well as the time between gas being accreted to the star-forming region and then ultimately forming stars (which allows for mixing within the disc). Significant numbers of stars are formed from gas that cycles back through the hot halo after first accreting to the star-forming region. Gas entering high-mass galaxies is pre-enriched in low-mass proto-galaxies prior to entering the virial radius of the central progenitor, with only small amounts of primordial gas accreted, even at high redshift (z ∼ 5). After entering the virial radius, significant further enrichment occurs prior to the accretion of the gas to the star-forming region, with gas that is feeding the star-forming region surpassing 0.1 Z⊙ by z = 0. Mixing with halo gas, itself enriched via galactic fountains, is thus crucial in determining the metallicity at which gas is accreted to the disc. The lowest mass simulated galaxy (Mvir ∼ 2 × 1010 M⊙, with M⋆ ∼ 107 M⊙), by contrast, accretes primordial gas through the virial radius and on to the disc, throughout its history. Much like the case for classical analytical solutions to the so-called ‘G-dwarf problem’, overproduction of low-metallicity stars is ameliorated by the interplay between the time of accretion on to the disc and the subsequent involvement in star formation – i.e. due to the inefficiency of star formation. Finally, gas outflow/metal removal rates from star-forming regions as a function of galactic mass are presented
New groups of planetary nebulae with peculiar dust chemistry towards the Galactic bulge
We investigate Galactic bulge planetary nebulae without emission-line central
stars for which peculiar infrared spectra have been obtained with the Spitzer
Space Telescope, including the simultaneous signs of oxygen and carbon based
dust. Three separate sub-groups can be defined characterized by the different
chemical composition of the dust and the presence of crystalline and amorphous
silicates.
We find that the classification based on the dust properties is reflected in
the more general properties of these planetary nebulae. However, some observed
properties are difficult to relate to the common view of planetary nebulae. In
particular, it is challenging to interpret the peculiar gas chemical
composition of many analyzed objects in the standard picture of the evolution
of planetary nebulae progenitors.
We confirm that the dual-dust chemistry phenomenon is not limited to
planetary nebulae with emission-line central stars.Comment: 17 pages, 13 figure
The Milky Way Bulge: Observed properties and a comparison to external galaxies
The Milky Way bulge offers a unique opportunity to investigate in detail the
role that different processes such as dynamical instabilities, hierarchical
merging, and dissipational collapse may have played in the history of the
Galaxy formation and evolution based on its resolved stellar population
properties. Large observation programmes and surveys of the bulge are providing
for the first time a look into the global view of the Milky Way bulge that can
be compared with the bulges of other galaxies, and be used as a template for
detailed comparison with models. The Milky Way has been shown to have a
box/peanut (B/P) bulge and recent evidence seems to suggest the presence of an
additional spheroidal component. In this review we summarise the global
chemical abundances, kinematics and structural properties that allow us to
disentangle these multiple components and provide constraints to understand
their origin. The investigation of both detailed and global properties of the
bulge now provide us with the opportunity to characterise the bulge as observed
in models, and to place the mixed component bulge scenario in the general
context of external galaxies. When writing this review, we considered the
perspectives of researchers working with the Milky Way and researchers working
with external galaxies. It is an attempt to approach both communities for a
fruitful exchange of ideas.Comment: Review article to appear in "Galactic Bulges", Editors: Laurikainen
E., Peletier R., Gadotti D., Springer Publishing. 36 pages, 10 figure
Stellar populations of bulges at low redshift
This chapter summarizes our current understanding of the stellar population
properties of bulges and outlines important future research directions.Comment: Review article to appear in "Galactic Bulges", Editors: Laurikainen
E., Peletier R., Gadotti D., Springer Publishing. 34 pages, 12 figure
Thin disc, Thick Disc and Halo in a Simulated Galaxy
Within a cosmological hydrodynamical simulation, we form a disc galaxy with
sub- components which can be assigned to a thin stellar disc, thick disk, and a
low mass stellar halo via a chemical decomposition. The thin and thick disc
populations so selected are distinct in their ages, kinematics, and
metallicities. Thin disc stars are young (<6.6 Gyr), possess low velocity
dispersion ({\sigma}U,V,W = 41, 31, 25 km/s), high [Fe/H], and low [O/Fe]. The
thick disc stars are old (6.6<age<9.8 Gyrs), lag the thin disc by \sim21 km/s,
possess higher velocity dispersion ({\sigma}U,V,W = 49, 44, 35 km/s),
relatively low [Fe/H] and high [O/Fe]. The halo component comprises less than
4% of stars in the "solar annulus" of the simulation, has low metallicity, a
velocity ellipsoid defined by ({\sigma}U,V,W = 62, 46, 45 km/s) and is formed
primarily in-situ during an early merger epoch. Gas-rich mergers during this
epoch play a major role in fuelling the formation of the old disc stars (the
thick disc). This is consistent with studies which show that cold accretion is
the main source of a disc galaxy's baryons. Our simulation initially forms a
relatively short (scalelength \sim1.7 kpc at z=1) and kinematically hot disc,
primarily from gas accreted during the galaxy's merger epoch. Far from being a
competing formation scenario, migration is crucial for reconciling the short,
hot, discs which form at high redshift in {\Lambda}CDM, with the properties of
the thick disc at z=0. The thick disc, as defined by its abundances maintains
its relatively short scale-length at z = 0 (2.31 kpc) compared with the total
disc scale-length of 2.73 kpc. The inside-out nature of disc growth is
imprinted the evolution of abundances such that the metal poor {\alpha}-young
population has a larger scale-length (4.07 kpc) than the more chemically
evolved metal rich {\alpha}-young population (2.74 kpc).Comment: Submitted to MNRAS. This version after helpful referee comments.
Comments welcome to [email protected]
Post conjunction detection of Pictoris b with VLT/SPHERE
With an orbital distance comparable to that of Saturn in the solar system,
\bpic b is the closest (semi-major axis \,9\,au) exoplanet that has
been imaged to orbit a star. Thus it offers unique opportunities for detailed
studies of its orbital, physical, and atmospheric properties, and of
disk-planet interactions. With the exception of the discovery observations in
2003 with NaCo at the Very Large Telescope (VLT), all following astrometric
measurements relative to \bpic have been obtained in the southwestern part of
the orbit, which severely limits the determination of the planet's orbital
parameters. We aimed at further constraining \bpic b orbital properties using
more data, and, in particular, data taken in the northeastern part of the
orbit.
We used SPHERE at the VLT to precisely monitor the orbital motion of beta
\bpic b since first light of the instrument in 2014. We were able to monitor
the planet until November 2016, when its angular separation became too small
(125 mas, i.e., 1.6\,au) and prevented further detection. We redetected \bpic b
on the northeast side of the disk at a separation of 139\,mas and a PA of
30 in September 2018. The planetary orbit is now well constrained.
With a semi-major axis (sma) of au (1 ), it
definitely excludes previously reported possible long orbital periods, and
excludes \bpic b as the origin of photometric variations that took place in
1981. We also refine the eccentricity and inclination of the planet. From an
instrumental point of view, these data demonstrate that it is possible to
detect, if they exist, young massive Jupiters that orbit at less than 2 au from
a star that is 20 pc away.Comment: accepted by A&
Planetary nebulae with emission-line central stars
The kinematic structure of a sample of planetary nebulae, consisting of 23
[WR] central stars, 21 weak emission line stars (wels) and 57 non-emission line
central stars, is studied. The [WR] stars are shown to be surrounded by
turbulent nebulae, a characteristic shared by some wels but almost completely
absent from the non-emission line stars. The fraction of objects showing
turbulence for non-emission-line stars, wels and [WR] stars is 7%, 24% and 91%,
respectively. The [WR] stars show a distinct IRAS 12-micron excess, indicative
of small dust grains, which is not found for wels. The [WR]-star nebulae are on
average more centrally condensed than those of other stars. On the
age-temperature diagram, the wels are located on tracks of both high and low
stellar mass, while [WR] stars trace a narrow range of intermediate masses.
Emission-line stars are not found on the cooling track. One group of wels may
form a sequence wels--[WO] stars with increasing temperature. For the other
groups both the wels and the [WR] stars appear to represent several,
independent evolutionary tracks. We find a discontinuity in the [WR] stellar
temperature distribution and suggest different evolutionary sequences above and
below the temperature gap. One group of cool [WR] stars has no counterpart
among any other group of PNe and may represent binary evolution. A prime factor
distinguishing wels and [WR] stars appears to be stellar luminosity. We find no
evidence for an increase of nebular expansion velocity with time.Comment: 14 pages, 9 figures, accepted to A&
The stellar halo of the Galaxy
Stellar halos may hold some of the best preserved fossils of the formation
history of galaxies. They are a natural product of the merging processes that
probably take place during the assembly of a galaxy, and hence may well be the
most ubiquitous component of galaxies, independently of their Hubble type. This
review focuses on our current understanding of the spatial structure, the
kinematics and chemistry of halo stars in the Milky Way. In recent years, we
have experienced a change in paradigm thanks to the discovery of large amounts
of substructure, especially in the outer halo. I discuss the implications of
the currently available observational constraints and fold them into several
possible formation scenarios. Unraveling the formation of the Galactic halo
will be possible in the near future through a combination of large wide field
photometric and spectroscopic surveys, and especially in the era of Gaia.Comment: 46 pages, 16 figures. References updated and some minor changes.
Full-resolution version available at
http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pd
Water in the terrestrial planet-forming zone of the PDS 70 disk
Terrestrial and sub-Neptune planets are expected to form in the inner
(AU) regions of protoplanetary disks. Water plays a key role in their
formation, although it is yet unclear whether water molecules are formed
in-situ or transported from the outer disk. So far Spitzer Space Telescope
observations have only provided water luminosity upper limits for dust-depleted
inner disks, similar to PDS 70, the first system with direct confirmation of
protoplanet presence. Here we report JWST observations of PDS 70, a benchmark
target to search for water in a disk hosting a large (AU)
planet-carved gap separating an inner and outer disk. Our findings show water
in the inner disk of PDS 70. This implies that potential terrestrial planets
forming therein have access to a water reservoir. The column densities of water
vapour suggest in-situ formation via a reaction sequence involving O, H,
and/or OH, and survival through water self-shielding. This is also supported by
the presence of CO emission, another molecule sensitive to UV
photodissociation. Dust shielding, and replenishment of both gas and small dust
from the outer disk, may also play a role in sustaining the water reservoir.
Our observations also reveal a strong variability of the mid-infrared spectral
energy distribution, pointing to a change of inner disk geometry.Comment: To appear in Nature on 24 July 2023. 21 pages, 10 figures; includes
extended data. Part of the JWST MINDS Guaranteed Time Observations program's
science enabling products. Spectra downloadable on Zenodo at
https://zenodo.org/record/799102
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