9 research outputs found
Velocity Dispersion Measurements of Milky Way Globular Clusters with VLT/X-shooter Spectroscopy
We have observed 29 globular clusters in the Magellanic Clouds and the Milky Way with VLT/X-shooter, a spectrograph with an exceptionally large spectral range from the ultraviolet to the near-infrared at moderately high resolution. The observations have been performed in drift-scan mode, where the telescope is slewed across the cluster during integration. Our comprehensive cascade of reduction steps allows for an uncertainty less than 10% in the absolute flux calibration and less than 0.02 angstrom in the wavelength calibration of the reduced spectra. For a subset of eleven clusters, for which accurate Hubble Space Telescope photometry is available, we construct detailed synthetic composite spectra based on their stellar populations, and subsequently use them as spectral templates to measure the velocity dispersion profile and radial velocity profile for each cluster. The obtained radial velocities indicate ordered rotation of some clusters. We use the central velocity dispersions to compute the dynamical masses and mass-to-light ratios for our sample. The sample median mass-to-light ratio is 1.7 M_sun / L_sun and fully consistent with a cluster mass that is entirely made up of stars and their remnants. In conjunction with our kinematic results follow-up numerical simulations will help to constrain the cluster mass profiles
The Panchromatic High-Resolution Spectroscopic Survey of Local Group Star Clusters - I. General Data Reduction Procedures for the VLT/X-shooter UVB and VIS arm
Our dataset contains spectroscopic observations of 29 globular clusters in
the Magellanic Clouds and the Milky Way performed with VLT/X-shooter. Here we
present detailed data reduction procedures for the VLT/X-shooter UVB and VIS
arm. These are not restricted to our particular dataset, but are generally
applicable to different kinds of X-shooter data without major limitation on the
astronomical object of interest. ESO's X-shooter pipeline (v1.5.0) performs
well and reliably for the wavelength calibration and the associated
rectification procedure, yet we find several weaknesses in the reduction
cascade that are addressed with additional calibration steps, such as bad pixel
interpolation, flat fielding, and slit illumination corrections. Furthermore,
the instrumental PSF is analytically modeled and used to reconstruct flux
losses at slit transit and for optimally extracting point sources. Regular
observations of spectrophotometric standard stars allow us to detect
instrumental variability, which needs to be understood if a reliable absolute
flux calibration is desired. A cascade of additional custom calibration steps
is presented that allows for an absolute flux calibration uncertainty of less
than ten percent under virtually every observational setup provided that the
signal-to-noise ratio is sufficiently high. The optimal extraction increases
the signal-to-noise ratio typically by a factor of 1.5, while simultaneously
correcting for resulting flux losses. The wavelength calibration is found to be
accurate to an uncertainty level of approximately 0.02 Angstrom. We find that
most of the X-shooter systematics can be reliably modeled and corrected for.
This offers the possibility of comparing observations on different nights and
with different telescope pointings and instrumental setups, thereby
facilitating a robust statistical analysis of large datasets.Comment: 22 pages, 18 figures, Accepted for publication in Astronomy &
Astrophysics; V2 contains a minor change in the abstract. We note that we did
not test X-shooter pipeline versions 2.0 or later. V3 contains an updated
referenc
2004 EW95: A Phyllosilicate-bearing Carbonaceous Asteroid in the Kuiper Belt
Models of the Solar System's dynamical evolution predict the dispersal of
primitive planetesimals from their formative regions amongst the gas-giant
planets due to the early phases of planetary migration. Consequently,
carbonaceous objects were scattered both into the outer asteroid belt and out
to the Kuiper Belt. These models predict that the Kuiper Belt should contain a
small fraction of objects with carbonaceous surfaces, though to date, all
reported visible reflectance spectra of small Kuiper Belt Objects (KBOs) are
linear and featureless. We report the unusual reflectance spectrum of a small
KBO, (120216) 2004 EW95, exhibiting a large drop in its near-UV reflectance and
a broad shallow optical absorption feature centered at ~700 nm which is
detected at greater than 4-sigma significance. These features, confirmed
through multiple epochs of spectral photometry and spectroscopy, have
respectively been associated with ferric oxides and phyllosilicates. The
spectrum bears striking resemblance to those of some C-type asteroids,
suggesting that 2004 EW95 may share a common origin with those objects. 2004
EW95 orbits the Sun in a stable mean motion resonance with Neptune, at
relatively high eccentricity and inclination, suggesting it may have been
emplaced there by some past dynamical instability. These results appear
consistent with the aforementioned model predictions and are the first to show
a reliably confirmed detection of silicate material on a small KBO.Comment: 8 pages, 3 figures, 1 table, accepted, in pres
2004 EW_(95): A Phyllosilicate-bearing Carbonaceous Asteroid in the Kuiper Belt
Models of the Solar System's dynamical evolution predict the dispersal of primitive planetesimals from their formative regions among the gas-giant planets due to the early phases of planetary migration. Consequently, carbonaceous objects were scattered both into the outer asteroid belt and out to the Kuiper Belt. These models predict that the Kuiper Belt should contain a small fraction of objects with carbonaceous surfaces, though to date, all reported visible reflectance spectra of small Kuiper Belt Objects (KBOs) are linear and featureless. We report the unusual reflectance spectrum of a small KBO, (120216) 2004 EW_(95), exhibiting a large drop in its near-UV reflectance and a broad shallow optical absorption feature centered at ~700 nm, which is detected at greater than 4Ï significance. These features, confirmed through multiple epochs of spectral photometry and spectroscopy, have respectively been associated with ferric oxides and phyllosilicates. The spectrum bears striking resemblance to those of some C-type asteroids, suggesting that 2004 EW_(95) may share a common origin with those objects. 2004 EW95 orbits the Sun in a stable mean motion resonance with Neptune, at relatively high eccentricity and inclination, suggesting it may have been emplaced there by some past dynamical instability. These results appear consistent with the aforementioned model predictions and are the first to show a reliably confirmed detection of silicate material on a small KBO
2004 EW_(95): A Phyllosilicate-bearing Carbonaceous Asteroid in the Kuiper Belt
Models of the Solar System's dynamical evolution predict the dispersal of primitive planetesimals from their formative regions among the gas-giant planets due to the early phases of planetary migration. Consequently, carbonaceous objects were scattered both into the outer asteroid belt and out to the Kuiper Belt. These models predict that the Kuiper Belt should contain a small fraction of objects with carbonaceous surfaces, though to date, all reported visible reflectance spectra of small Kuiper Belt Objects (KBOs) are linear and featureless. We report the unusual reflectance spectrum of a small KBO, (120216) 2004 EW_(95), exhibiting a large drop in its near-UV reflectance and a broad shallow optical absorption feature centered at ~700 nm, which is detected at greater than 4Ï significance. These features, confirmed through multiple epochs of spectral photometry and spectroscopy, have respectively been associated with ferric oxides and phyllosilicates. The spectrum bears striking resemblance to those of some C-type asteroids, suggesting that 2004 EW_(95) may share a common origin with those objects. 2004 EW95 orbits the Sun in a stable mean motion resonance with Neptune, at relatively high eccentricity and inclination, suggesting it may have been emplaced there by some past dynamical instability. These results appear consistent with the aforementioned model predictions and are the first to show a reliably confirmed detection of silicate material on a small KBO