Gaia DR2

Accumulation of objects in the firmament.
Color coding: very few very many objects per square degree

Gaia DR2 is a star catalog published by DPAC on April 25, 2018 with around 1.7 billion objects based on the observations made by the Gaia spacecraft . Gaia DR2 is an abbreviation for Gaia Data Release2 and is the second major release of the Gaia mission after Gaia DR1 . In addition to star data, Gaia DR2 also contains information on binary stars , quasars and asteroids .

Data basis

Card of stars with colors. Each bright point corresponds to a catalog entry, the color corresponds to the data of the photometer.

The first star catalog Gaia DR1 was based on the first 14 months of the observation period. Initially, a prepared object list, the Initial Gaia Source List (IGSL), was used, which was compiled from various already existing catalogs. For part of Gaia DR1, data from the Tycho 2 catalog and the Hipparcos catalog , which are based on measurements from the astrometry satellite Hipparcos , were combined with Gaia measurements. The result is called Tycho-Gaia Astrometric Solution (TGAS).

The data from DR2 are based exclusively on Gaia's observations from 22 months of observation from July 25, 2014 to May 23, 2016. The initial object lists and the results of TGAS were not used in DR2. For the calibration of the magnitudes, the Gaia Spectrophotometric Standard Star Catalog (SPSS) was used, which is maintained in this function during the entire mission.

DR2 is a stand-alone publication of Gaia data only, based on principles and principles other than DR1, and should therefore not be viewed as a newer version of the Gaia DR1 catalog. A new object list was created from the collected data, the Main Data Base Catalog (MDB) with around 2.5 billion objects, which serves as the basis for DR2. The objects were given new object IDs. Objects from DR1 can no longer be present in DR2, become several separate objects or several objects can be merged to form a new object. These effects can be caused, for example, by duplicate entries and other errors in the IGSL. The newly generated object list prevents the continuation of problems of any kind from the IGSL or the catalogs on which the IGSL is based. From this list only those objects were published that meet the quality criteria, in particular that have a sufficient number of observations.

For Gaia DR2 51 billion registered objects and 520 billion astrometric measurements from the astrometric field as well as 102 billion low-resolution spectra from the two photometers were evaluated.

Magnitudes

For the measurement of the apparent brightness (magnitudes) of the objects, a different photometric system was used than with DR1. The pass bands , which are determined by the spectral sensitivities of the detectors and the transmittivities of the filters and other optical components, have been recalibrated. The measured brightnesses are referred to as G _BP for measurements with the blue photometer (wavelength range 330–680 nm) and G _RP for the red photometer (wavelength range 640–1000 nm). In addition, there is the unfiltered G-band magnitude (G), which covers the entire wavelength range from 350 to 1000 nm. A measurement of G, G _BP and G _RP together give a three-band photometry. The radial velocity spectrometer (RVS) determines an additional magnitude G _RVS for the small spectral range from 845 to 872 nm for lighter objects , but G _RVS has not yet been sufficiently calibrated so that these values have not yet been published in DR2.

Comparison between different catalogs

There is a possibility to track IDs from DR1 and DR2. There is a comparison with objects from Hipparcos-2 , Tycho-2 , 2MASS Point Source Catalogs , SDSS DR9, Pan-STARRS , GSC2.3 , PPM-XL, AllWISE and URAT-1.

All data from DR2 has been available on the Internet since April 25, 2018 via any of the five data centers of the Gaia archive. There are no restrictions on the use of this data. The original data is available as CSV and unsorted. The right ascension applies to 2015.5. An unofficial source provides the data in tabular form in 1-degree zones and sorted by right ascension calculated back to 2000.0.

Result

Objects and data in DR2, visualization. Source: ESA

The result of Gaia DR2
Type of objects	number	available data	Measurement uncertainty	other uncertainties
Objects with G-band magnitude overall.	1,692,919,135	Position (α, δ), G-band magnitude (G)	general systematic uncertainty <0.1 mas (averaged)	G magnitude (mmag) G <13: 0.3 G = 17: 2 G = 20:10
Objects with only two parameters up to G ≈ 21	361.009.408	Position, G-band magnitude. Mostly faint objects.	Position (mas) G = 20: 2 (J2015.5)	G magnitude (mmag) G <13: 0.3 G = 17: 2 G = 20:10
Objects with data from the blue photometer (BP)	1,381,964,755	position G-band magnitude G _BP Magnitude / G _RP Magnitude		G _BP and G _{RP (mmag)} G <13: 2 G = 17:10 G = 20: 200
Red photometer data (RP)	1,383,551,713			G _BP and G _{RP (mmag)} G <13: 2 G = 17:10 G = 20: 200
Objects with five parameters, Magnitude from G = 21 to G ≈ 3	1,331,909,727	Position (α, δ), G-band magnitude parallax ${\ displaystyle \ varpi}$ Angular velocity (μ _α , μ _δ),	Position and parallax (mas) G <15: 0.02-0.04 G = 17: 0.1 G = 20: 0.7 G = 21: 2 Proper movement (mas / year) G <15: 0.07 G = 17: 0.2 G = 20: 1.2 G = 21: 3
Of which objects with radial velocity (six parameters) Magnitude GRVS = 4 to GRVS ≈ 13. Temperature range (T _eff ) from 3550 to 6900 K	7,224,631	Position (α, δ) G-band magnitude parallax ${\ displaystyle \ varpi}$ Angular velocity (μ _α , μ _δ), Radial velocity		Radial speed (km / s) GRVS <8: 0.3 GRVS = 10: 0.6 GRVS = 11.75: 1.8
Variable objects with light curves	550.737	Cepheids , RR Lyrae , Mira and semi-regular candidates , high-amplitude Delta Scuti , BY Draconis candidates , SX Phoenicis candidates , short-term phenomena.
Known asteroids	14,099	Epochs All data from the observation of asteroids and other solar system objects are forwarded to the Minor Planet Center (MPC).	position 96% in the range of ± 5 mas 52% in the range of ± 1 mas, each in the scanning direction
Objects of the Gaia CRF2	556,869	Quasars for the reference frame (Gaia Celestial Reference Frame)	Position (mas) G <18: 0.12 G <20: 0.5 G ≥ 20: 0.8 systematic uncertainty 20-30 μas	rotation <10 μas / year
ICRF3 objects	2,820			rotation <10 μas / year
Effective temperature T _eff	161,497,595		324 K
Extinction A _G and color index E (G _BP -G _RP )	87,733,672	Color index G _BP - G _RP , blue objects have positive values, red objects have negative values.	A _G: 0.46 mag E: 0.23 mag
Radius (R) and luminosity (L)	76,956,778		R = 10% L = 15%

Gaia Celestial Reference Frame (GCRF2)

Gaia DR2 contains the positions of 556,869 quasars with G magnitude 16 to 21. They form their own reference system for measuring star locations. The zero point for this measuring system is the barycenter of the solar system, the axis of rotation is fixed in relation to these distant objects and corresponds to that of the ICRF . For Gaia DR2, this frame of reference was compared with a preliminary version of the ICRF3 . A subset of common 2820 objects corresponds very well in position to the pre-release version of ICRF3. ${\ displaystyle \ simeq}$

In contrast to the other objects, the quasars of Gaia DR1 and DR2 have not yet been identified and compared using the photometric data, but rather using existing catalogs. The preliminary version of ICRF3 and the AllWISE AGN catalog containing 1.4 million objects were used for the comparison, which shows active galactic nuclei visibly in the mid-infrared range. So far, Gaia has been able to find an optical equivalent at least once for around half of the objects.

Catalog of Radial Velocity Standard Stars (RVSS)

The RSV instrument does not have its own calibration facility. For DR2 the radial velocities were calibrated on the basis of a collection of stars whose radial velocities were already known through previous terrestrial observations. These stars must hardly have any changes in brightness , must not be close to other similar stars in their position and must be within a certain magnitude limit and cover the sky evenly. There was no catalog that covered stars with these conditions, so a catalog had to be created for this purpose, the Catalog of Radial Velocity Standard Stars (RVSS).

A first list, valid at the time of the start of Gaia, contained 1420 standard stars, which can also be found in the Hipparcos catalog, these had a known radial speed with an accuracy of 300 m / s based on measurement series. Based on the data models, it was found that Gaia would measure an average of one of these stars per hour, but at least two standard stars per hour were required for a satisfactory calibration. A new list now contains 4813 standard stars, 2712 of which now met the stricter condition of accuracy to a maximum of 100 m / s, so these were selected for the calibration, the other stars on the list were used to validate the results. In order to have enough measurements for each of these stars, an observation program was started. Five observatories with specialized instruments for precise measurement of radial velocities were involved in this program: SOPHIE , HARPS , ELODIE , CORALIE , and NARVAL on the Pic du Midi de Bigorre . For the zero point determination, asteroids with very precisely known orbital data were selected. Thanks to the Doppler effect that can be calculated with this , the zero point can be determined very precisely.

Known limits

The database's object list contains approximately 2.5 billion objects. Gaia DR2, despite all the improvements over Gaia DR1, is still a preliminary catalog on a limited database. Not all of the data and objects collected up to that point were actually evaluated and published. Some of the data was filtered out before publication if the amount of data was insufficient or the values deviated too much.

completeness

The catalog is largely complete in the magnitude range between G = 12 and G = 17. The completeness for light objects has been improved, but some of the objects for G≲7 are missing. At the weaker end there is no clear definition of the magnitude limit. The limit depends heavily on the sky position. With a density of over 400,000 stars per square degree , the limit is approximately G = 18. Since the number of observations plays a role, the density distribution of the objects is still not uniform. The detection of objects near very bright objects has been improved, but is not yet optimal. Stars with a very high proper motion could be captured better, but about 20% of the objects with proper motion> 0.6 arcsec / year are missing. The angular resolution has improved compared to DR1 to 0.4 arcsec.

Astrometry

There are systematic errors depending on the sky position, magnitude and color, which are estimated to be less than 0.1 mas. There is an average parallax zero point of -30 μas. A small number of objects with disturbed parallaxes can be identified by unusually large positive or negative values. The astrometric uncertainties are mathematically derived from the data and have not been calibrated on external data . The values are about 7-10% underestimated for faint objects with G> 16 outside the galactic plane and about 30% for stars with G <12. The quality of the astrometric data for objects with a magnitude of brighter than 6 is generally weaker than that other data. Most of the 361 million objects with only two parameters are on the faint end of the magnitude scale. A study found that some of the bright objects with G <5 can contain large systematic errors in the information on parallax due to a calibration problem.

Photometry

The results of the photometry are impaired at the faint end at G> 19. Factors are the scattered light problem of the probe, the assessment of the background brightness is not precise enough and the data has not yet been specially treated to eliminate the effects of overlay and nearby bright objects. By definition, G = G _BP + G _RP . However, there are inconsistent luminous flux measurements where the luminous flux of G does not match the sum of G _BP and G _RP . The deviation from this value is expressed using the flux excess factor.

Radial velocity

Radial velocities (RV) have not yet been determined for metal-rich stars with strong overlapping emission lines . Overlapping spectroscopic binary stars are treated like single stars and only an average radial velocity is given. Radial velocities have not yet been calculated for some types of binary stars, such objects are missing.

In 2019 Douglas Boubert et al. a problem with measurements of radial velocities. On closer examination it turned out that with some of the values the spectra of neighboring bright objects are superimposed and thus falsified the results. The problem is most common in densely populated areas and a list has been drawn up listing objects that may be affected by the problem. The list includes 70,365 potentially contaminated objects, including 4,000 with clearly incorrect radial velocities, including all with an RV greater than 625 km / s. In these cases, the data of a nearby object with a comparable brightness was incorrectly included, resulting in extremely enlarged values. Gaia EDR3 will have an updated list of GDR2's objects with the 4000 objects with incorrect values removed.

Astrophysical values

The values for T _eff , extinction A _G , redness E (G _BP -G _RP ) or color, radius and luminosity were derived solely from three-band photometry in connection with the parallax for individual stars. The connection between temperature, extinction and redness cannot be sufficiently resolved with this broadband measurement, so that many assumptions were made in order to estimate these values. We recommend that you consult the accompanying documentation before using these values.

Variable stars

Only some of the variable stars have been explored in detail. Not all variable stars have been classified and some of the classification may be incorrect.

Solar System Objects (SSO)

The objects are not completely based on one criterion; a preselection of well-known main belt asteroids , near-earth objects and Trojans has been published . All published objects had at least 9 transits. Light objects with G≲10 were removed because the values were influenced by systematic effects and are not yet precisely calibrated. G-band photometry is available for roughly half of the transits. The objects that Gaia can observe are, with a few exceptions, asteroids, so that the term SSO almost coincides with asteroid. The automatic object recognition for SSO has not yet been used for DR2; this will only play a role in later publications. For DR2, the objects found during the Initial Data Treatment (IDT) were compared with the expected transit data of all known objects as they are registered in the Minor Planet Center .

Catalog error

The catalog errors were only discovered after publication. Approximately nine million objects were incorrectly divided between objects with two parameters and objects with five parameters. Around two million objects were published with five parameters, although the data basis was actually insufficient, and about seven million were published with two parameters, although the data would have been sufficient for a publication with five parameters.

Milestones

The first Gaia DR1 catalog with 1.1 billion objects was published on September 14, 2016.
The second catalog Gaia DR2 with 1.7 billion objects on April 25, 2018
The third catalog Gaia EDR3 with around 1.8 billion as the first part of the third publication, planned for the end of 2020.

At the time of publication of Gaia DR2, all raw data from 34 months of observation time had already been collected for the Gaia DR3 catalog. The Gaia DR3 catalog was originally due to be released in the first half of 2021. It became foreseeable that the parts of DR3 will be ready for publication at different times. So that the data are available to science as early as possible, DR3 is to be published in two parts.

The first part under the name Gaia EDR3 should come out in the third quarter of 2020 and contain improved astrometry and photometry such as: B. star locations, parallaxes, proper motion and three-band photometry. Gaia DR3 with the rest of the more complex data is expected to come out in the second half of 2021. In addition to the data from EDR3, DR3 should also provide the spectroscopic and photometric object classifications for easily evaluable objects, RVS spectra and information on the stellar atmosphere, radial velocities, classifications for variable stars with photometric curves, catalogs of objects of the solar system with preliminary orbital data and individual data about their observations included, as well as catalogs of quasars, extended objects, and multiple stars. DR3 is to contain an additional catalog with all objects from an area in a radius of 5.5 ° around the Andromeda galaxy. This catalog, called Andromeda Photometric Survey (GAPS), is said to contain around 1 million objects. The Covid-19 pandemic also delayed work on Gaia DR3, so it is not certain that the release dates can be kept.

Gaia DR4 should contain the results of the nominal mission duration with the observation data until July 15, 2019. With the increasing duration of the mission, the originally planned dates for the publications were withdrawn. The publication of the data obtained during the nominal duration of the mission was not expected until the end of 2022 at the earliest, now no date has been given. This includes all astrometric and photometric data, all variable stars, all double and multiple star systems, classifications and various astrometric data for stars, unresolved double stars, galaxies and quasars, a list of exoplanets, all epochs and transit dates for all objects.Template: future / in 2 years

The official extension of the mission brings with it further publications. The latest data will be published approximately three years after the mission ends.

literature

Gaia Collaboration: Gaia Data Release 2; Documentation release 1.2 . Ed .: European Space Agency and Gaia Data Processing and Analysis Consortium. June 5, 2019 ( esa.int [PDF]).
Gaia Collaboration: Gaia Data Release 2; Summary of the contents and survey properties . Ed .: Astronomy & Astrophysics. April 16, 2018, doi : 10.1051 / 0004-6361 / 201833051 .

Web links

Gaia DR2 (Gaia Collaboration, 2018): Description at VizieR
Gaia Archives. Retrieved April 30, 2018 . , online portal for the retrieval of data.
Gaia Data Release 2, Citation Rules. Retrieved April 30, 2018 .
Gaia DR2 on VizieR. Retrieved May 7, 2018 .
Gaia for Guide. Retrieved July 7, 2018 .
DR2 papers. Retrieved August 11, 2019 .

Individual evidence

↑ Gaiaverse: The first Gaia catalog "Gaia DR1" provided by Genius Project
↑ ^a ^b ^c D. Katz, AGA Brown: Gaia: on the road to DR2 . October 30, 2017, arxiv : 1710.10816v2 .
↑ M. Riello et al .: Gaia Data Release 2; Processing of the photometric data . August 2018, doi : 10.1051 / 0004-6361 / 201832712 ( aanda.org [PDF]).
↑ Gaia DR2 passbands. Retrieved April 12, 2018 (UK English).
↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Gaia DR2 - Cosmos. Retrieved April 10, 2018 (UK English).
↑ GAIA DATA ACCESS. Retrieved April 21, 2018 .
↑ Gaia Archives. Retrieved August 6, 2017 .
↑ Gaia for Guide. (No longer available online.) Archived from the original on July 7, 2018 ; accessed on July 7, 2018 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2
↑ Gaia Collaboration: Gaia Data Release 2; Summary of the contents and survey properties . Ed .: Astronomy & Astrophysics. April 16, 2018, p. 2 , doi : 10.1051 / 0004-6361 / 201833051 .
^ ^A ^b F. Mignard, SA Klioner, L. Lindegren, J. Hernández, U. Bastian: Gaia Data Release 2 - The celestial reference frame (Gaia-CRF2) . In: Astronomy & Astrophysics . tape 616 , August 1, 2018, ISSN 0004-6361 , p. A14 , doi : 10.1051 / 0004-6361 / 201832916 ( aanda.org [accessed August 31, 2020]).
↑ ^a ^b Gaia Collaboration: F. Mignard, S. Klioner, L. Lindegren, J. Hernandez, U. Bastian, A. Bombrun et al .: Gaia Data Release 2; The Celestial reference frame (Gaia-CRF2) . Ed .: A&A. April 25, 2018, arxiv : 1804.09377 ( arxiv.org [PDF]).
↑ ALLWISEAGN - AllWISE Catalog of Mid-IR AGNs. Retrieved July 7, 2018 .
↑ F. Crifo, G. Jasniewicz, C. Soubiran, D. Katz, A. Siebert: Towards a new full-sky list of radial velocity standard stars . In: Astronomy & Astrophysics . tape 524 , December 1, 2010, ISSN 0004-6361 , p. A10 , doi : 10.1051 / 0004-6361 / 201015315 ( aanda.org [accessed August 28, 2020]).
^ ^A ^b C. Soubiran, G. Jasniewicz, L. Chemin, C. Zurbach, N. Brouillet: Gaia Data Release 2 - The catalog of radial velocity standard stars . In: Astronomy & Astrophysics . tape 616 , August 1, 2018, ISSN 0004-6361 , p. A7 , doi : 10.1051 / 0004-6361 / 201832795 ( aanda.org [accessed August 28, 2020]).
↑ Ronald Drimmel, Beatrice Bucciarelli, Laura Inno: Possible Large Systematic Errors of Gaia DR2 Parallaxes for Very Bright Stars . In: Research Notes of the AAS . tape 3 , no. 6 , June 6, 2019, ISSN 2515-5172 , p. 79 , doi : 10.3847 / 2515-5172 / ab2632 .
↑ Douglas Boubert et al .: Lessons from the curious case of the `` fastest 'star in Gaia DR2 . January 28, 2019, arxiv : 1901.10460 .
↑ ^a ^b List of objects with problematic radial velocities . January 28, 2019, arxiv : 1901.10460v1 .
↑ Gaia DR2 known issues - Cosmos. Retrieved November 29, 2018 (UK English).
↑ ^a ^b Data Release scenario. ESA, 2017, accessed September 4, 2017 .
↑ News 2019 - Gaia - Cosmos. Retrieved October 1, 2019 .
↑ News 2020 - Gaia - Cosmos. Retrieved March 25, 2020 .

[1] Gaiaverse: The first Gaia catalog "Gaia DR1" provided by Genius Project

[:2-2] D. Katz, AGA Brown: Gaia: on the road to DR2 . October 30, 2017, arxiv : 1710.10816v2 .

[3] M. Riello et al .: Gaia Data Release 2; Processing of the photometric data . August 2018, doi : 10.1051 / 0004-6361 / 201832712 ( aanda.org [PDF]).

[4] Gaia DR2 passbands. Retrieved April 12, 2018 (UK English).

[:0-5] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Gaia DR2 - Cosmos. Retrieved April 10, 2018 (UK English).

[6] GAIA DATA ACCESS. Retrieved April 21, 2018 .

[7] Gaia Archives. Retrieved August 6, 2017 .

[8] Gaia for Guide. (No longer available online.) Archived from the original on July 7, 2018 ; accessed on July 7, 2018 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2

[9] Gaia Collaboration: Gaia Data Release 2; Summary of the contents and survey properties . Ed .: Astronomy & Astrophysics. April 16, 2018, p. 2 , doi : 10.1051 / 0004-6361 / 201833051 .

[:5-10] A ^b F. Mignard, SA Klioner, L. Lindegren, J. Hernández, U. Bastian: Gaia Data Release 2 - The celestial reference frame (Gaia-CRF2) . In: Astronomy & Astrophysics . tape 616 , August 1, 2018, ISSN 0004-6361 , p. A14 , doi : 10.1051 / 0004-6361 / 201832916 ( aanda.org [accessed August 31, 2020]).

[:1-11] Gaia Collaboration: F. Mignard, S. Klioner, L. Lindegren, J. Hernandez, U. Bastian, A. Bombrun et al .: Gaia Data Release 2; The Celestial reference frame (Gaia-CRF2) . Ed .: A&A. April 25, 2018, arxiv : 1804.09377 ( arxiv.org [PDF]).

[12] ALLWISEAGN - AllWISE Catalog of Mid-IR AGNs. Retrieved July 7, 2018 .

[13] F. Crifo, G. Jasniewicz, C. Soubiran, D. Katz, A. Siebert: Towards a new full-sky list of radial velocity standard stars . In: Astronomy & Astrophysics . tape 524 , December 1, 2010, ISSN 0004-6361 , p. A10 , doi : 10.1051 / 0004-6361 / 201015315 ( aanda.org [accessed August 28, 2020]).

[:3-14] A ^b C. Soubiran, G. Jasniewicz, L. Chemin, C. Zurbach, N. Brouillet: Gaia Data Release 2 - The catalog of radial velocity standard stars . In: Astronomy & Astrophysics . tape 616 , August 1, 2018, ISSN 0004-6361 , p. A7 , doi : 10.1051 / 0004-6361 / 201832795 ( aanda.org [accessed August 28, 2020]).

[15] Ronald Drimmel, Beatrice Bucciarelli, Laura Inno: Possible Large Systematic Errors of Gaia DR2 Parallaxes for Very Bright Stars . In: Research Notes of the AAS . tape 3 , no. 6 , June 6, 2019, ISSN 2515-5172 , p. 79 , doi : 10.3847 / 2515-5172 / ab2632 .

[16] Douglas Boubert et al .: Lessons from the curious case of the `` fastest 'star in Gaia DR2 . January 28, 2019, arxiv : 1901.10460 .

[:4-17] List of objects with problematic radial velocities . January 28, 2019, arxiv : 1901.10460v1 .

[18] Gaia DR2 known issues - Cosmos. Retrieved November 29, 2018 (UK English).

[releasescenario-19] Data Release scenario. ESA, 2017, accessed September 4, 2017 .

[20] News 2019 - Gaia - Cosmos. Retrieved October 1, 2019 .

[21] News 2020 - Gaia - Cosmos. Retrieved March 25, 2020 .