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Register a new userThe parallax zero point offset from Gaia EDR3 data
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Martin Groenewegen
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The second data release of it Gaia rm revealed a parallax zero point offset of $-0.029$~mas based on quasars. The value depended on the position on the sky, and also likely on magnitude and colour. The offset and its dependence on other parameters inhibited an improvement in the local distance scale using e.g. the Cepheid and RR Lyrae period-luminosity relations. Analysis of the recent it Gaia rm Early Data Release 3 (EDR3) reveals a mean parallax zero point offset of $-0.021$~mas based on quasars. The it Gaia rm team addresses the parallax zero point offset in detail and proposes a recipe to correct for it, based on ecliptic latitude, $G$-band magnitude, and colour information. This paper is a completely independent investigation into this issue focussing on the spatial dependence of the correction based on quasars and the magnitude dependence based on wide binaries. The spatial and magnitude corrections are connected to each other in the overlap region between $17 < G < 19$. The spatial correction is presented at several spatial resolutions based on the HEALPix formalism. The colour dependence of the parallax offset is unclear and in any case secondary to the spatial and magnitude dependence. The spatial and magnitude corrections are applied to two samples of brighter sources, namely a sample of $sim$100 stars with independent trigonometric parallax measurements from it HST rm data, and a sample of 75 classical cepheids using photometric parallaxes. The mean offset between the observed GEDR3 parallax and the independent trigonometric parallax (excluding outliers) is about $-39$~muas, and after applying the correction it is consistent with being zero. For the classical cepheid sample it is suggested that the photometric parallaxes may be underestimated by about 5%.
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We independently determine the zero-point offset of the Gaia early Data Release-3 (EDR3) parallaxes based on $sim 110,000$ W Ursae Majoris (EW)-type eclipsing binary systems. EWs cover almost the entire sky and are characterized by a relatively complete coverage in magnitude and color. They are an excellent proxy for Galactic main-sequence stars. We derive a $W1$-band Period-Luminosity relation with a distance accuracy of $7.4%$, which we use to anchor the Gaia parallax zero-point. The final, global parallax offsets are $-28.6pm0.6$ $mu$as and $-25.4pm4.0$ $mu$as (before correction) and $4.2pm0.5$ $mu$as and $4.6pm3.7$ $mu$as (after correction) for the five- and six-parameter solutions, respectively. The total systematic uncertainty is $1.8$ $mu$as. The spatial distribution of the parallax offsets shows that the bias in the corrected Gaia EDR3 parallaxes is less than 10 $mu$as across $40%$ of the sky. Only $15%$ of the sky is characterized by a parallax offset greater than 30 $mu$as. Thus, we have provided independent evidence that the parallax zero-point correction provided by the Gaia team significantly reduces the prevailing bias. Combined with literature data, we find that the overall Gaia EDR3 parallax offsets for Galactic stars are $[-20, -30]$ $mu$as and 4-10 $mu$as, respectively, before and after correction. For specific regions, an additional deviation of about 10 $mu$as is found.
Using offset-corrected Gaia-EDR3 parallax measurements and spectrophotometric methods, we have determined distances for 69 massive stars in the Carina OB1 association and associated clusters: Trumpler 16 (21 stars), Trumpler 14 (20 stars), Trumpler 15 (3 stars), Bochum 11 (5 stars), and South Pillars region (20 stars). Past distance estimates to the Carina Nebula range from 2.2 to 3.6 kpc, with uncertainties arising from photometry and anomalous dust extinction. The EDR3 parallax solutions show considerable improvement over DR2, with typical errors $sigma_{varpi}/varpi approx$~3-5%. The O-type stars in the Great Carina Nebula lie at essentially the same distance ($2.35pm0.08$ kpc), quoting mean and rms variance. The clusters have distances of $2.32pm0.12$ kpc (Tr 16), $2.37pm0.15$ kpc (Tr 14), $2.36pm0.09$ kpc (Tr 15), and $2.33pm0.12$ kpc (Bochum 11) in good agreement with the $eta$ Car distance of around 2.3 kpc. O-star proper motions suggest internal (2D) velocity dispersions $sim4$ km/s for Tr 14 and Tr 16. Reliable distances allow estimates of cluster sizes, stellar dynamics, luminosities, and fluxes of photoionizing radiation incident on photodissociation regions in the region. We estimate that Tr 14 and Tr 16 have half-mass radii $r_h = 1.5-1.8$ pc, stellar crossing times $t_{rm cr} = r_h/v_m approx 0.7-0.8$ Myr, and two-body relaxation times $t_{rh} approx 40-80$ Myr. The underlying velocity dispersion for Tr 14, if a bound cluster, would be $v_m approx 2.1^{+0.7}_{-0.4}$ km/s for $N = 7600^{+5800}_{-2600}$ stars. With the higher dispersions of the O-stars, mass segregation might occur slowly, on times scales of 3-6~Myr.
Using the recent GAIA eDR3 catalogue we construct a sample of solar neighbourhood isolated wide binaries satisfying a series of strict signal-to-noise data cuts, exclusion of random association criteria and detailed colour-magnitude diagram selections, to minimise the presence of any kinematic contaminating effects having been discussed in the literature to date. Our final high-purity sample consists of 421 binary pairs within 130 pc of the sun and in all cases high-quality GAIA single-stellar fits for both components of each binary (final average RUWE values of 0.99), both also restricted to the cleanest region of the main sequence. We find kinematics fully consistent with Newtonian expectations for separations, $s$, below 0.009 pc, with relative velocities scaling with $Delta V propto s^{-1/2}$ and a total binary mass, $M_{b}$, velocity scaling of $Delta V propto M_{b}^{1/2}$. For the separation region of $s> 0.009$ pc we obtain significantly different results, with a separation independent $Delta V approx 0.5$ km/s and a $Delta V propto M_{b}^{0.22 pm 0.18}$. This situation is highly reminiscent of the low acceleration galactic baryonic Tully-Fisher phenomenology, and indeed, the change from the two regimes we find closely corresponds to the $a lesssim a_{0}$ transition.
We present a cross-calibration of Hipparcos and Gaia EDR3 intended to identify astrometrically accelerating stars and to fit orbits to stars with faint, massive companions. The resulting catalog, the EDR3 edition of the Hipparcos-Gaia Catalog of Accelerations (HGCA), provides three proper motions with calibrated uncertainties on the EDR3 reference frame: the Hipparcos proper motion, the Gaia EDR3 proper motion, and the long-term proper motion given by the difference in position between Hipparcos and Gaia EDR3. Our approach is similar to that for the Gaia DR2 edition of the HGCA, but offers a factor of ~3 improvement in precision thanks to the longer time baseline and improved data processing of Gaia EDR3. We again find that a 60/40 mixture of the two Hipparcos reductions outperforms either reduction individually, and we find strong evidence for locally variable frame rotations between all pairs of proper motion measurements. The substantial global frame rotation seen in DR2 proper motions has been removed in EDR3. We also correct for color- and magnitude-dependent frame rotations at a level of up to ~50 $mu$as/yr in Gaia EDR3. We calibrate the Gaia EDR3 uncertainties using a sample of radial velocity standard stars without binary companions; we find an error inflation factor (a ratio of total to formal uncertainty) of 1.37. This is substantially lower than the position dependent factor of ~1.7 found for Gaia DR2 and reflects the improved data processing in EDR3. While the catalog should be used with caution, its proper motion residuals provide a powerful tool to measure the masses and orbits of faint, massive companions to nearby stars.
The early third data release (EDR3) of the European Space Agency satellite Gaia provides coordinates, parallaxes, and proper motions for ~1.47 billion sources in our Milky Way, based on 34 months of observations. The combination of Gaia DR2 radial velocities with the more precise and accurate astrometry provided by Gaia EDR3 makes the best dataset available to search for the fastest nearby stars in our Galaxy. We compute the velocity distribution of ~7 million stars with precise parallaxes, to investigate the high-velocity tail of the velocity distribution of stars in the Milky Way. We release a catalogue with distances, total velocities, and corresponding uncertainties for all the stars considered in our analysis, available at https://sites.google.com/view/tmarchetti/research . By applying quality cuts on the Gaia astrometry and radial velocities, we identify a clean subset of 94 stars with a probability Pub > 50% to be unbound from our Galaxy. 17 of these have Pub > 80% and are our best candidates. We propagate these stars in the Galactic potential to characterize their orbits. We find that 11 stars are consistent with being ejected from the Galactic disk, and are possible hyper-runaway star candidates. The other 6 stars are not consistent with coming from a known star-forming region. We investigate the effect of adopting a parallax zero point correction, which strongly impacts our results: when applying this correction, we identify only 12 stars with Pub > 50%, 3 of these having Pub > 80%. Spectroscopic follow-ups with ground-based telescopes are needed to confirm the candidates identified in this work.
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