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Gaia news portal

The news in this bulletin board is compiled from various sources across the world wide web.
From the Gaia Image of the Week page: Gaia's CCD detectors are covered with a thin film. To optimise the transmission of light from the vacuum of space into the silicon of the light detectors, the silicon is covered with a very thin anti-reflective (AR) coating. The thickness chosen for that layer depends on the wavelength range which is to be observed with a particular CCD detector: for the CCDs of Gaia's "Blue Spectrometer" (BP), designed to obtain spectra of stars in the short-wavelength, blue to ultraviolet part of the spectrum, this layer is the thinnest. For the CCDs used in the "Astrometric Field" (AF), designed to determine the position of stars from as much light as possible, all the way from ultraviolet to infrared, the anti-reflective coating is thicker. For the CCDs of Gaia's "Red Photometer" (RP) and "Radial Velocity Spectrometer", both used for producing spectra at longer wavelengths in the red and near-infrared, the film is the thickest.

02/10/2015: The colour of Gaia's eyes

From the Gaia Image of the Week page: Gaia's CCD detectors are covered with a thin film. To optimise the transmission of light from the vacuum of space into the silicon of the light detectors, the silicon is covered with a very thin anti-reflective (AR) coating. The thickness chosen for that layer depends on the wavelength range which is to be observed with a particular CCD detector: for the CCDs of Gaia's "Blue Spectrometer" (BP), designed to obtain spectra of stars in the short-wavelength, blue to ultraviolet part of the spectrum, this layer is the thinnest. For the CCDs used in the "Astrometric Field" (AF), designed to determine the position of stars from as much light as possible, all the way from ultraviolet to infrared, the anti-reflective coating is thicker. For the CCDs of Gaia's "Red Photometer" (RP) and "Radial Velocity Spectrometer", both used for producing spectra at longer wavelengths in the red and near-infrared, the film is the thickest. [more]
From the Gaia Image of the Week page: Parallax is defined as the reciprocal of distance. But when we measure a parallax, this measurement is always noisy, and its simple reciprocal is not necessarily a good estimate of the distance. In fact, it has been known for some time that once the fractional parallax error is more than about 20%, the reciprocal parallax is a rather poor and biased estimate of the distance. This is an issue for Gaia, because about 80% of all stars which Gaia observes will have fractional parallax errors larger than 20%.
This fact can be seen in the above plots, which both show the cumulative distribution of fractional parallaxes. The black line in the right panel shows the fraction of Gaia stars (vertical axis) which are expected to have a fractional parallax error below the value given on the horizontal axis. This was calculated using the GUMS catalogue and the sky-averaged, post-launch, Gaia astrometric accuracy model assuming five years of observations. The red line is for the actual fractional parallax errors from the Hipparcos mission. Hipparcos generally has a larger fraction of stars with a smaller given fractional error than Gaia, but Hipparcos observed a far smaller absolute number of stars and with larger absolute parallax errors. The left panel shows the same information but on a logarithmic scale.

24/09/2015: Estimating distances from parallaxes

From the Gaia Image of the Week page: Parallax is defined as the reciprocal of distance. But when we measure a parallax, this measurement is always noisy, and its simple reciprocal is not necessarily a good estimate of the distance. In fact, it has been known for some time that once the fractional parallax error is more than about 20%, the reciprocal parallax is a rather poor and biased estimate of the distance. This is an issue for Gaia, because about 80% of all stars which Gaia observes will have fractional parallax errors larger than 20%.

This fact can be seen in the above plots, which both show the cumulative distribution of fractional parallaxes. The black line in the right panel shows the fraction of Gaia stars (vertical axis) which are expected to have a fractional parallax error below the value given on the horizontal axis. This was calculated using the GUMS catalogue and the sky-averaged, post-launch, Gaia astrometric accuracy model assuming five years of observations. The red line is for the actual fractional parallax errors from the Hipparcos mission. Hipparcos generally has a larger fraction of stars with a smaller given fractional error than Gaia, but Hipparcos observed a far smaller absolute number of stars and with larger absolute parallax errors. The left panel shows the same information but on a logarithmic scale.

[more]
From ESA's Gaia site: Last Friday, 21 August, ESA’s billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode.
After launch on 19 December 2013 and a six-month long in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. By repeatedly measuring the positions of the stars with extraordinary accuracy, Gaia can tease out their distances and motions through the Milky Way galaxy.

25/08/2015: Gaia's first year of scientific observations

From ESA's Gaia site: Last Friday, 21 August, ESA’s billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode.

After launch on 19 December 2013 and a six-month long in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. By repeatedly measuring the positions of the stars with extraordinary accuracy, Gaia can tease out their distances and motions through the Milky Way galaxy.

[more]
From the Gaia news page: The Hertzsprung-Russell (HR) diagrams presented here were produced as part of a first validation of the astrometric capabilities of the instrument and of the Basic Angle variation measured by the on-board interferometer (Basic Angle Monitor; BAM). Based on less than one year of data from the routine observation phase of Gaia, they give an exciting hint of what the mission will deliver.

07/08/2015: Demonstrating Gaia's astrometric potential

From the Gaia news page: The Hertzsprung-Russell (HR) diagrams presented here were produced as part of a first validation of the astrometric capabilities of the instrument and of the Basic Angle variation measured by the on-board interferometer (Basic Angle Monitor; BAM). Based on less than one year of data from the routine observation phase of Gaia, they give an exciting hint of what the mission will deliver. [more]

29/07/2015: The Gaia-ESO Survey: Characterisation of the [alpha/Fe] sequences in the Milky Way discs

ArXiv:1507.08066 by G. Kordopatis et al: We investigate, using the Gaia-ESO Survey internal Data-Release 2, the properties of the double sequence of the Milky Way discs (defined chemically as the high-alpha and low-alpha populations), and discuss their compatibility with discs defined by other means such as metallicity, kinematics or positions.

This investigation uses two different approaches: in velocity space for stars located in the extended Solar neighbourhood, and in chemical space for stars at different ranges of Galactocentric radii and heights from the plane. The separation we find in velocity space allows us to investigate, in a novel manner, the extent in metallicity of each of the two sequences, identifying them with the two discs, without making any assumption about the shape of their metallicity distribution functions. Then, using the separation in chemical space, we characterise the spatial variation of the slopes of the [alpha/Fe] - [Fe/H] sequences for the thick and thin discs and the way in which the relative proportions of the two discs change across the Galaxy. We find that the thick disc (high-alpha sequence), extends up to [Fe/H]~ +0.2 and the thin disc (low-alpha sequence), at least down to [Fe/H]~ -0.8. Radial and vertical gradients in alpha-abundances are found for the thin disc, with mild spatial variations in its [alpha/Fe] - [Fe/H] paths, whereas for the thick disc we do not detect any such spatial variations.

The small variations in the spatial [alpha/Fe] - [Fe/H] paths of the thin disc do not allow us to distinguish between formation models of this structure. On the other hand, the lack of radial gradients and [alpha/Fe] - [Fe/H] variations for the thick disc indicate that the mechanism responsible for the mixing of the metals in the young Galaxy (e.g. radial stellar migration or turbulent gaseous disc) was more efficient before the (present) thin disc started forming.

[more]
From the Gaia Image of the Week page: Gaia observes more than a billion stars on the whole sky, without knowing in advance where they are. However, as each source is observed multiple times, the Initial Data Processing (IDT, a highly sophisticated piece of software running on the data transmitted by the satellite, developed by the University of Barcelona team) has the task of grouping together multiple observations of the same source.
This task, the so-called "cross-matching", involves comparing the positions recorded by Gaia. If two sources are observed, within the uncertainty, at the same position on the sky they are recognized to be – in fact – the same source.
For asteroids, this cannot work, as they are always moving amongst the stars - slowly (typically, an asteroid in the Main Belt can take a couple of days to move a distance of a Moon diameter) but fast enough for Gaia (several pixels during a single transit on the focal plane)! As a result, Gaia never sees an asteroid at the same place, and the cross-matching described above leaves these detections as "orphans" that do not repeat over time.

31/07/2015: Asteroids all around

From the Gaia Image of the Week page: Gaia observes more than a billion stars on the whole sky, without knowing in advance where they are. However, as each source is observed multiple times, the Initial Data Processing (IDT, a highly sophisticated piece of software running on the data transmitted by the satellite, developed by the University of Barcelona team) has the task of grouping together multiple observations of the same source.

This task, the so-called "cross-matching", involves comparing the positions recorded by Gaia. If two sources are observed, within the uncertainty, at the same position on the sky they are recognized to be – in fact – the same source.

For asteroids, this cannot work, as they are always moving amongst the stars - slowly (typically, an asteroid in the Main Belt can take a couple of days to move a distance of a Moon diameter) but fast enough for Gaia (several pixels during a single transit on the focal plane)! As a result, Gaia never sees an asteroid at the same place, and the cross-matching described above leaves these detections as "orphans" that do not repeat over time.

[more]
From the Gaia Image of the Week page: An international team of researchers, with the assistance of amateur astronomers, have discovered a unique binary star system: the first known such system where one star completely eclipses the other. It is a type of two-star system known as a Cataclysmic Variable, where one super dense white dwarf star is stealing gas from its companion star, effectively ``cannibalising'' it.

17/07/2015: Gaia satellite and amateur astronomers spot one in a billion star

From the Gaia Image of the Week page: An international team of researchers, with the assistance of amateur astronomers, have discovered a unique binary star system: the first known such system where one star completely eclipses the other. It is a type of two-star system known as a Cataclysmic Variable, where one super dense white dwarf star is stealing gas from its companion star, effectively ``cannibalising'' it. [more]

15/07/2015: Detection of satellite remnants in the Galactic Halo with Gaia III. Detection limits for Ultra Faint Dwarf Galaxies

arXiv:1507.04353 by T. Antoja et al: We present a method to identify Ultra Faint Dwarf Galaxy (UFDG) candidates in the halo of the Milky Way using the future Gaia catalogue and we explore its detection limits and completeness. The method is based on the Wavelet Transform and searches for over-densities in the combined space of sky coordinates and proper motions, using kinematics in the search for the first time. We test the method with a Gaia mock catalogue that has the Gaia Universe Model Snapshot (GUMS) as a background, and use a library of around 30 000 UFDGs simulated as Plummer spheres with a single stellar population. For the UFDGs we use a wide range of structural and orbital parameters that go beyond the range spanned by real systems, where some UFDGs may remain undetected. We characterize the detection limits as function of the number of observable stars by Gaia in the UFDGs with respect to that of the background and their apparent sizes in the sky and proper motion planes. We find that the addition of proper motions in the search improves considerably the detections compared to a photometric survey at the same magnitude limit. Our experiments suggest that Gaia will be able to detect UFDGs that are similar to some of the known UFDGs even if the limit of Gaia is around 2 magnitudes brighter than that of SDSS, with the advantage of having a full-sky catalogue. We also see that Gaia could even find some UFDGs that have lower surface brightness than the SDSS limit. [more]
 
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