The Gaia Ultra-Cool Dwarf Sample -- II: Structure at the end of the main sequence

TL;DR

This paper compiles a detailed sample of late M, L, and T dwarfs from Gaia DR2, analyzing their properties, multiplicity, and photometric behavior to improve understanding of ultra cool dwarfs at the end of the main sequence.

Contribution

It provides a new, well-characterized sample of 695 ultra cool dwarfs, including new multiple systems, and offers calibration and analysis of their photometric and spectral properties.

Findings

100 objects are in 47 multiple systems, with 20 new discoveries.

GBP magnitudes are unreliable for these objects.

Main sequence scatter increases with redder colors.

Abstract

We identify and investigate known late M, L and T dwarfs in the Gaia second data release. This sample is being used as a training set in the Gaia data processing chain of the ultra cool dwarfs work package. We find 695 objects in the optical spectral range M8 to T6 with accurate Gaia coordinates, proper motions, and parallaxes which we combine with published spectral types and photometry from large area optical and infrared sky surveys. We find that 100 objects are in 47 multiple systems, of which 27 systems are published and 20 are new. These will be useful benchmark systems and we discuss the requirements to produce a complete catalog of multiple systems with an ultra cool dwarf component. We examine the magnitudes in the Gaia passbands and find that the GBP magnitudes are unreliable and should not be used for these objects. We examine progressively redder colour magnitude diagrams and see a notable increase in the main sequence scatter and a bivariate main sequence for old and young objects. We provide an absolute magnitude spectral sub-type calibration for G and GRP passbands along with linear fits over the range M8 L8 for other passbands.

Tables

Table 1. Table 1: Input catalog entries with multiple Gaia  DR2 matches within 20 20 20 ″. Where both objects are in the input catalog of UCDs we include their names. Objects with “Bin” in the first column are found to be physical binaries based on the test discussed in Section  3 .

Short	Discovery Name	Offset	$G$	$G-G_{\mathrm{est}}$	$\varpi$	${\mu }_{\mathrm{tot}}$	${\theta }_{\mu }$
name	and Source_ID	″	mag	mag	mas	mas yr-1	°
J0004-4044	GJ 1001 B	0.8	18.353	-0.4	82.1 $\pm$ 0.4	1641.6	155.9
Bin	4996141155411983744	18.4	11.500	-7.2	81.2 $\pm$ 0.1	1650.7	155.8
J0235-2331	GJ 1048 B	0.1	18.598	0.6	46.6 $\pm$ 0.3	97.0	77.5
	5125414998097353600	12.1	7.987	-10.1	47.1 $\pm$ 0.0	84.5	80.4
J0858+2710	2MASS 08583693+2710518	0.1	19.926	0.3	18.9 $\pm$ 1.3	221.4	155.9
Bin	692611481331037952	15.2	15.067	-4.6	17.9 $\pm$ 0.1	215.0	156.8
J1004+5022	G 196-3 B	0.2	20.170	0.3	44.4 $\pm$ 0.8	250.0	213.5
Bin	824017070904063104	15.9	10.612	-9.3	45.9 $\pm$ 0.0	246.8	214.9
J1004-3335	2MASSWJ1004392-333518	0.3	19.615	0.3	53.3 $\pm$ 0.6	495.1	135.7
Bin	5458784415381054464	12.0	12.908	-6.5	53.5 $\pm$ 0.1	488.8	135.0
J1047+4046	LP213-067	4.4	15.183	-1.3	40.1 $\pm$ 0.1	299.9	263.7
J1047+4047	LP213-068	4.5	16.931	-0.7	38.9 $\pm$ 0.5	303.3	263.5
J1202+4204	2MASS 12025009+4204531	0.2	19.321	-0.5	31.5 $\pm$ 0.4	366.6	217.5
Bin	1537249785437526784	7.8	16.430	-3.4	31.6 $\pm$ 0.1	368.9	218.3
J1219+0154	ULAS J121932.54+015433.0	0.1	19.792	-0.6	18.9 $\pm$ 0.6	114.9	229.9
Bin	3700975728440669184	10.9	13.441	-6.9	19.8 $\pm$ 0.1	115.2	230.4
J1245+0156	ULAS J124531.54+015630.9	0.1	20.612	-0.5	13.5 $\pm$ 1.2	76.0	234.7
Bin	3702489721592680832	8.2	12.860	-8.3	13.2 $\pm$ 0.0	75.6	235.1
J1304+0907	2MASS 13043318+0907070	0.1	20.173	-0.4	18.2 $\pm$ 0.8	134.8	278.6
Bin	3734192764990097408	7.6	15.160	-5.4	17.8 $\pm$ 0.1	134.3	277.9
J1442+6603A	G 239-25 A	0.2	9.851	-2.3	91.5 $\pm$ 0.0	301.6	262.6
J1442+6603	G 239-25 B	0.2	15.302	-1.4	91.7 $\pm$ 0.2	338.5	274.3
J1520-4422	WDS J15200-4423A	0.4	18.293	-0.3	54.5 $\pm$ 0.2	736.7	238.6
J1520-4422B	WDS J15200-4423B	0.4	19.817	1.0	53.7 $\pm$ 0.6	753.4	238.6
J1540+0102	ULAS J154005.10+010208.7	0.0	19.851	-0.7	14.8 $\pm$ 0.6	51.7	253.1
	4416887712294719104	13.6	14.863	-5.6	17.0 $\pm$ 0.7	50.9	267.0
J1711+4028	G 203-50 B	5.5	20.232	-0.4	47.4 $\pm$ 0.7	263.5	72.4
Bin	1341903196663707904	8.7	14.233	-6.4	47.1 $\pm$ 0.1	265.5	72.0
J2200-3038A	DENIS-PJ220002.05-303832.9A	3.7	18.437	-0.3	25.4 $\pm$ 0.4	247.2	104.9
J2200-3038B	DENIS-PJ220002.05-303832.9B	0.1	19.042	-0.6	25.3 $\pm$ 0.5	253.7	105.6
J2308+0629	ULAS J230818.73+062951.4	0.1	18.059	-0.7	24.7 $\pm$ 0.3	118.5	162.3
Bin	2665079816223169664	3.8	13.467	-5.3	24.1 $\pm$ 0.1	119.8	160.5
J2322-6151	2MASS 23225299-6151275	0.0	20.682	0.3	23.2 $\pm$ 1.0	114.6	135.7
Bin	6487249243899899904	16.6	14.902	-5.5	23.6 $\pm$ 0.1	110.3	135.2

Table 2. Table 4: Objects with published parallax estimates differing by more than 2.5 σ 𝜎 \sigma with the Gaia  DR2 parallax.

Short	Gaia $\varpi$	Published $\varpi$
Name	mas	mas
J0439-2353	80.79 $\pm$ 0.51	110.40 $\pm$ 4.001
J0445-3048	61.97 $\pm$ 0.18	78.50 $\pm$ 4.901
J0615-0100	44.80 $\pm$ 0.33	45.70 $\pm$ 0.112
J0805+4812	46.78 $\pm$ 0.96	43.10 $\pm$ 1.003
J1017+1308	34.56 $\pm$ 0.82	30.20 $\pm$ 1.403
J1155-3727	84.57 $\pm$ 0.19	104.38 $\pm$ 4.691
J1207-3932A	15.52 $\pm$ 0.16	19.10 $\pm$ 0.404
J1254-0122	74.18 $\pm$ 2.31	84.90 $\pm$ 1.905
J1359-4034	47.51 $\pm$ 0.27	64.18 $\pm$ 5.481
J1454-6604	93.22 $\pm$ 0.30	84.88 $\pm$ 1.716
J1506+7027	193.55 $\pm$ 0.94	310.00 $\pm$ 42.007
J1610-0040	29.14 $\pm$ 0.37	31.02 $\pm$ 0.268
J1717+6526	46.86 $\pm$ 0.62	57.05 $\pm$ 3.519
J1731+2721	83.74 $\pm$ 0.12	113.80 $\pm$ 7.0010
J1807+5015	68.33 $\pm$ 0.13	77.25 $\pm$ 1.489
J2148+4003	123.28 $\pm$ 0.46	101.01 $\pm$ 1.7811

Table 3. Table 6: J1711+5430 and NLTT 44368 Gaia  DR2 parameters.

Name	$\varpi$	${\mu }_{\alpha }\cos \delta$	${\mu }_{\delta }$
	mas	mas yr-1	mas yr-1
J1711+5430	22.06 $\pm$ 0.60	-48.71 $\pm$ 1.70	206.73 $\pm$ 1.904
NLTT 44368	21.14 $\pm$ 0.04	-61.62 $\pm$ 0.12	211.31 $\pm$ 0.092

Equations

$$r=\frac{{\varpi }_N-{\varpi }_P}{\sqrt{{\sigma }_N^2+{\sigma }_p^2}},$$

$$\begin{array}{rl}\rho & <100\varpi ,\\ \Delta \varpi & <\max [1.0,3{\sigma }_{\varpi }],\\ \Delta \mu & <0.1\mu ,\\ \Delta \theta & <15\text{degr},\end{array}$$

$$M_{\lambda }=a_{\lambda }+b_{\lambda }\mathrm{SpT}$$

Full text

1 Introduction

The Gaia second data release (hereafter Gaia DR2; Gaia Collaboration et al., 2018a) was made on April 25th 2018 and contains parallaxes, proper motions, and magnitudes for over one billion objects. The main astrometric observations use a large optical passband called the $G$ band, and the completeness magnitude goal of this mission in this band is 20.7 mag (Gaia Collaboration et al., 2016). We are interested in ultra-cool dwarfs (hereafter UCDs), defined as objects with a spectral type later than M7. UCDs are intrinsically very faint in the optical and, therefore, only limited numbers will be observable by Gaia. In particular, we expect there to be around a 1000 L dwarfs and only a few T dwarfs (Haywood & Jordi, 2002; Sarro et al., 2013; Smart, 2014; Smart et al., 2017).

While this sample is relatively limited in numbers, the availability of all-sky uniformly derived parallaxes provides a volume limited sample that is very useful for a number of astrophysical problems. Gaia UCDs include objects with masses that straddle the stellar–sub-stellar transition, and therefore help us define the observational boundary between hydrogen-burning stars and degenerate brown dwarfs (e.g. see Chabrier et al., 2009; Burrows et al., 2011). The final volume limited sample will be used to model the stellar–sub-stellar mass function (Allen et al., 2005) and luminosity function (Cruz et al., 2007), removing incompleteness and observational biases (e.g. Malmquist, Eddington and Lutz-Kelker effects) that plague current measurements of this fundamental observable (e.g. see Kirkpatrick et al., 2012; Marocco et al., 2015, and references therein).

The Gaia astrometry and photometry will provide robust measurements of luminosity. The Gaia dataset will aid the modelling of the atmospheres of low-mass objects by providing a cohort of new benchmark systems, such as companions to main-sequence stars (Marocco et al., 2017; Montes et al., 2018) and members of young moving groups (e.g. Gagné et al., 2015). L dwarfs are analogues for understanding planetary atmospheres (Faherty et al., 2016) and, once we calibrate a cooling curve (e.g. by studying L dwarf companions to white dwarfs; Day-Jones et al., 2011), their ubiquity will make them promising Galactic chronometers (Soderblom, 2010; Burgasser, 2009).

A first step in identifying Gaia L and T (hereafter LT) dwarfs was carried out in Smart et al. (2017, hereafter Paper 1), matching known LT dwarfs to the first Gaia data release (Gaia Collaboration et al., 2016), which contained accurate positions and $G$ magnitudes for 1.14 billion objects. This cross-match resulted in 321 LT dwarfs with Gaia $G$ magnitudes and positions. This catalogue makes up the cool part of the Gaia Ultra-cool Dwarf Sample (hereafter GUCDS), which is being used as a training set in Coordination Unit 8 of the Gaia Data Processing and Analysis Consortium pipeline111https://www.cosmos.esa.int/web/gaia/coordination-units. In addition, Gaia Collaboration et al. (2018b) cross-matched the input catalogue from Paper 1 with Gaia DR2 and external catalogues such as 2MASS (Skrutskie et al. 2006). This exercise provided 601 LT dwarfs, including 527 fully characterised objects. Here, we build on this legacy and carry out a more comprehensive analysis.

In this paper we concentrate on the LT dwarfs that are in the Gaia DR2. In Section 2 we describe the input catalogue of LT dwarfs used to search the Gaia DR2, the cleaning carried out, and the production of the LT part of the GUCDS catalogue. In Section 3 we look at LT dwarfs that are in binary systems with other objects in the Gaia DR2. In Section 4 we examine this catalogue in absolute magnitude, colour, and spectroscopic space. In the last section we give conclusions and future plans.

2 The comparison catalogues

2.1 The Gaia DR2 Selection

Each of the 1332 million Gaia DR2 sources with full astrometric solutions are the result of individual five-parameter fits to their epoch positions. It is inevitable that some of these fits produce physically nonsensical solutions with large negative parallaxes being the most obvious examples. The solutions with large positive parallaxes that appear to be nearby objects represent the tail of the $10^{9}$ solutions distribution and is, in a relative sense, significantly impacted by objects being scattered into that solution space. Indeed, if one orders Gaia DR2 by parallax, Proxima Centauri, the closest object to the Sun, would be ranked 61st. If we consider objects with parallaxes greater than 200 mas (i.e. distance $d<$ 5 pc), there are 792 of them in the Gaia DR2. However, only 38 have a parallax in SIMBAD (Wenger et al., 2000) that is greater than 200 mas. There are also 34 of the 792 objects that match to SIMBAD entries but have parallaxes or photometric distances that place them at distances greater than 5 pc. While there is a remote possibility that some of the new objects with parallaxes greater than 200 mas in the Gaia DR2 are really within 5 pc, the majority, if not all, of the remaining 754 solutions are incorrect.

In Lindegren et al. (2018, their Appendix C) they convincingly argued that many of these bad solutions are due to mismatches of the observations. They showed, as it would be expected if this is the dominant reason, that the number of objects with large negative parallaxes is approximately equal to the number of sources with large spurious positive parallaxes. They also provided a number of quality cuts that would reduce the contamination at a small cost to the identification of real objects. However, as our final goal is to make a complete census of all UCDs in the Gaia dataset, we want our training set to include also objects with low quality astrometry so we do not apply those cuts. In addition, the Gaia DR2 is missing astrometric solutions for prominent nearby bright LT dwarfs, e.g. $WISE$ J1049-5319A, (Luhman, 2013) and $\epsilon$ Indi B ab (Scholz et al., 2003), probably because these are binary systems with large orbital motions and their solutions did not meet the quality thresholds for inclusion in the Gaia DR2.

Since the majority of large parallaxes are unreliable and some of the nearest objects are missing, it is premature to attempt to find all UCDs to the Gaia magnitude limit and, therefore, we concentrate on developing criteria for a robust selection procedure in the future. The first step in developing such criteria is the identification of known UCDs that we can use as a training set. In Paper 1 we showed that the most distant single L0 that we expected to see in Gaia is at 80 pc. There are unresolved binary L dwarf systems outside the 100 pc limit that have a combined magnitude greater than the Gaia DR2 limit. There are also very young L dwarfs that have very bright intrinsic magnitudes for their spectral type and these may enter the Gaia DR2 even though they are at a distance greater than 100 pc. For example some of the L dwarfs identified in the Upper Scorpius OB association (see Lodieu et al., 2008, and reference therein) at a distance of 145$\pm$2 pc (de Zeeuw et al., 1999) with an age of 5 Myr (Preibisch & Zinnecker, 1999) have predicted Gaia apparent magnitudes $G<20$ mag from Paper 1. However, the vast majority of LT dwarfs seen by Gaia are within 80 pc, so we start by selecting all objects from the Gaia DR2 with a parallax greater than 10 mas, e.g. a distance limit of 100 pc, which results in 700,055 sources.

2.2 The M, L, T or Y catalogue