302 research outputs found
The radial metallicity gradients in the Milky Way thick disk as fossil signatures of a primordial chemical distribution
In this letter we examine the evolution of the radial metallicity gradient
induced by secular processes, in the disk of an -body Milky Way-like galaxy.
We assign a [Fe/H] value to each particle of the simulation according to an
initial, cosmologically motivated, radial chemical distribution and let the
disk dynamically evolve for 6 Gyr. This direct approach allows us to take into
account only the effects of dynamical evolution and to gauge how and to what
extent they affect the initial chemical conditions. The initial [Fe/H]
distribution increases with R in the inner disk up to R ~ 10 kpc and decreases
for larger R. We find that the initial chemical profile does not undergo major
transformations after 6 Gyr of dynamical evolution. The final radial chemical
gradients predicted by the model in the solar neighborhood are positive and of
the same order of those recently observed in the Milky Way thick disk.
We conclude that: 1) the spatial chemical imprint at the time of disk
formation is not washed out by secular dynamical processes, and 2) the observed
radial gradient may be the dynamical relic of a thick disk originated from a
stellar population showing a positive chemical radial gradient in the inner
regions.Comment: 10 pages, 5 figures, Accepted for publication on Astrophysical
Journal Letter
Laboratory Characterisation of a Commercial RGB CMOS Camera for Measuring Night Sky Brightness
The use of RGB cameras in photometric applications has grown over the last few decades in many fields such as industrial applications, light engineering and the analysis of the quality of the night sky. In this last field, they are often used in conjunction with a Sky Quality Meter (SQM), an instrument used for the measurement of night sky brightness (NSB), mainly when there is a significant amount of artificial light at night (ALAN). The performances of these two instruments are compared here. A simple source composed of nine narrowband LEDs in an integrating sphere was used to excite the two instruments and therefore measure the spectral responsivity of the SQM and of the three channels of the camera. The estimated uncertainties regarding spectral responsivity were less than 10%. A synthetic instrument approximating the SQM's responsivity can be created using a combination of the R, G and B channels. The outputs of the two instruments were compared by measuring the spectral radiance of the night sky. An evaluation of the spectral mismatch between the two instruments completed the analysis of their spectral sensitivity. Finally, the measurements of real SQMs in four sites experiencing different levels of light pollution were compared with the values obtained by processing the recorded RGB images. Overall, the analysis shows that the two instruments have significantly different levels of spectral responsivity, and the alignment of their outputs requires the use of a correction which depends on the spectral distribution of the light coming from the sky. A synthetic SQM will always underestimate real SQM measures; an average correction factor was evaluated considering nine sky spectra under low and medium levels of light pollution; this was determined to be 1.11 and, on average, compensated for the gap. A linear correction was also supposed based on the correlation between the NSB levels measured by the two instruments; the mean squared error after the correction was 0.03 mag arcsec-2
The Overdensity in Virgo, Sagittarius Debris, and the Asymmetric Spheroid
We investigate the relationship between several previously identified
Galactic halo stellar structures in the direction of Virgo using imaging and
spectroscopic observations of F turnoff stars and blue horizontal branch stars
from the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic
Understanding and Exploration (SEGUE). We show that the Sagittarius dwarf
leading tidal tail does not pass through the solar neighborhood; it misses the
Sun by more than 15 kpc, passing through the Galactic plane outside the Solar
Circle. It also is not spatially coincident with the large stellar overdensity
S297+63-20.5 in the Virgo constellation. S297+63-20.5 has a distinct turnoff
color and kinematics. Faint (g ~ 20.3) turnoff stars in S297+63-20.5 have
line-of-sight, Galactic standard of rest velocities V(GSR)= 130 +/- 10 km/s,
opposite in sign to infalling Sgr tail stars. The path of the Sgr leading tidal
tail is also inconsistent with the positions of some of the nearer stars with
which it has been associated, and whose velocities have favored models with
prolate Milky Way potentials. We additionally show that the number densities of
brighter (g ~ 19.8) F turnoff stars are not symmetric about the Galactic
center, and that this discrepancy is not primarily due to the S297+63-20.5
moving group. Either the spheroid is asymmetric about the Galactic center, or
there are additional substructures that conspire to be on the same side of the
Galaxy as S297+63-20.5. The S297+63-20.5 overdensity in Virgo is likely
associated with two other previously identified Virgo substructures: the Virgo
Stellar Stream (VSS) and the Virgo Overdensity (VOD). However, the velocity
difference between the VSS and S297+63-20.5 and the difference in distance
estimates between the VOD and S297+63-20.5 must be reconciled.Comment: 10 figures, ApJ in pres
SÃndrome da hipertensão pulmonar: a ascite em frangos de corte.
bitstream/item/57929/1/CUsersPiazzonDocumentsCIT-27.pd
Sky Quality Meter and satellite correlation for night cloud-cover analysis at astronomical sites
The analysis of night cloud cover is very important for astronomical observations in real time, considering a typical observation time of about 15 minutes, and to provide statistics. In this article, we use the Sky Quality Meter (SQM) for high-resolution temporal analysis of the La Silla and Asiago (Ekar Observatory) sky: 3 and 5 minutes respectively. We investigate the annual temporal evolution of the natural contributions of the sky at a site not influenced by artificial light at night (ALAN) and at one highly influenced. We also make a correlation between GOES and Aqua satellite data and ground-based SQM data to confirm the relationship between the SQM data and cloud cover. We develop an algorithm that allows the use of the SQM for night cloud detection and reach correlations with the nighttime cloud cover detected by the GOES and Aqua satellites of 97.2 per cent at La Silla and 94.6 per cent at Asiago. Our algorithm also classifies photometric (PN) and spectroscopic nights (SN). We measure 59.1 per cent PN and 21.7 per cent SN for a total percentage of clear nights of 80.8 per cent at La Silla in 2018. The respective Ekar Observatory values are 31.1 per cent PN, 24.0 per cent SN and 55.1 per cent of total clear night time. Application to the SQM network would involve the development of long-term statistics and large data forecasting models for site testing and real-time astronomical observation
A new kinematic model of the Galaxy: analysis of the stellar velocity field from Gaia Data Release 3
This work presents the results of a kinematic analysis of the Galaxy that
uses a new model as applied to the newest available Gaia data. We carry out the
Taylor decomposition of the velocity field up to second order for 18 million
high luminosity stars (i.e. OBAF-type stars, giants and subgiants) from the
Gaia DR3 data. We determine the components of mean stellar velocities and their
first and second partial derivatives (relative to cylindrical coordinates) for
more than 28 thousand points in the plane of our Galaxy. We estimate Oort's
constants , , , and and other kinematics parameters and map them
as a function of Galactocentric coordinates. The values found confirm the
results of our previous works and are in excellent agreement with those
obtained by other authors in the Solar neighbourhood. In addition, the
introduction of second order partial derivatives of the stellar velocity field
allows us to determine the values of the vertical gradient of the Galaxy
azimuthal, radial and vertical velocities. Also, we determine the mean of the
Galaxy rotation curve for Galactocentric distances from 4 kpc to 18 kpc by
averaging Galactic azimuths in the range -30 < < +30
about the direction Galactic Centre-Sun-Galactic anticentre. Maps of the
velocity components and of their partial derivatives with respect to
coordinates within 10 kpc of the Sun reveal complex substructures, which
provide clear evidence of non-axisymmetric features of the Galaxy. Finally, we
show evidence of differences in the Northern and Southern hemispheres stellar
velocity fields.Comment: 19 pages, 14 figures, 2 table
The chemical signature of the Galactic spiral arms revealed by Gaia DR3
Taking advantage of the recent Gaia Data Release 3 (DR3), we map chemical
inhomogeneities in the Milky Way's disc out to a distance of 4 kpc from
the Sun, using different samples of bright giant stars. The samples are
selected using effective temperatures and surface gravities from the GSP-Spec
module, and are expected to trace stellar populations of different typical age.
The cool (old) giants exhibit a relatively smooth radial metallicity gradient
with an azimuthal dependence. Binning in Galactic azimuth , the slope
gradually varies from [M/H] dex kpc at to dex kpc at . On the
other hand, the relatively hotter (and younger) stars present remarkable
inhomogeneities, apparent as three (possibly four) metal-rich elongated
features in correspondence of the spiral arms' locations in the Galactic disc.
When projected onto Galactic radius, those features manifest themselves as
statistically significant bumps on top of the observed radial metallicity
gradients with amplitudes up to dex, making the assumption of
a linear radial decrease not applicable to this sample. The strong correlation
between the spiral structure of the Galaxy and the observed chemical pattern in
the young sample indicates that the spiral arms might be at the origin for the
detected chemical inhomogeneities. In this scenario, the spiral arms would
leave in the younger stars a strong signature, which progressively disappears
when cooler (and older) giants are considered.Comment: 9 pages, 6 figures. Accepted for publication in A&A Letter
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