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BY 4.0 license Open Access Published by De Gruyter Open Access February 16, 2023

Crossmatching of high-proper motion L, T, and Y brown dwarfs with large photometric surveys

  • Aleksandra S. Avdeeva , Sergey V. Karpov and Oleg Yu. Malkov EMAIL logo
From the journal Open Astronomy

Abstract

L, T, and Y brown dwarf (BD) exploration is a topical issue. First, the transition boundary between planets and BDs is not fully understood yet. Second, there is a phenomenon called the L/T-transition. The observed properties of cooler BDs differ significantly from the less cool ones, namely, colors of BDs shift much bluer and, simultaneously, dwarfs become brighter with a decrease in temperature. Models of this phenomenon are now being actively developed, for which reason BDs observed in a wide range of spectra are needed. Our goal is to refine the position of BDs in the parameter space of colors and magnitudes in order to effectively search for them in photometric surveys (without involving spectroscopic observations). For that reason, we crossmatched about 500 known L, T, and Y BDs with large photometric surveys. We paid special attention to the problem of identifying objects near the Sun because the crossmatching by default is made by searching for the closest object in a particular survey, which can be inappropriate concerning the high proper motions of our dwarfs. We have determined the color indices that can be used to search for these objects in large photometric surveys, and we have pointed out the color indices that are inappropriate for this purpose. We also made preliminary estimates of the radius matching to search for objects in the modern photometric surveys, given their proximity to the Sun and, consequently, their high proper motion.

1 Introduction

Crossmatching of various information sources is a powerful tool in astronomy that helps to not only enrich and augment the contents of individual ones but also discover new and unique objects. In astronomy, cross-matching of catalogs is a standard tool for obtaining broader information on the objects by combining their data from the surveys performed at different wavelengths, and it allows us to solve a number of tasks like studying various populations of astronomical objects or investigating the properties of the interstellar medium. Crossmatching of high-proper motion objects is an especially tricky problem because crossmatching is usually performed with respect to the positions of objects in two different surveys.

An example of high proper motion object is brown dwarfs (BDs), which are close to the Sun. BDs are sub-stellar objects. Their mass is not enough to start and maintain stable hydrogen fusion, which causes them to cool over time. In the spectral classification, BDs have spectral types L, T, and Y. BDs have low temperatures, so they are quite weak in the visible spectrum, and the peak of the radiation intensity is in the infrared range. As these objects are relatively weak, most of them are comparatively close to the Sun and thus are having high proper motions and are complicated to crossmatch.

According to the studies of Mužić et al. (2017), the number of BDs in the Galaxy ranges from 25 to 100 billion objects, with the total number of objects ranging from 100 to 500 billion. However, few of them are discovered. Homogeneous and complete samples of BDs are needed for various kinds of studies: kinematic studies, studies of binary stars with BDs, and studies of the parameters of the Galaxy. To determine astrophysical parameters of BDs from their observed quantities, it is also necessary to refine the dependencies spectral type – luminosity and spectral type – density of objects in the nearest neighborhood.

Numerous attempts have been made to search for and create a set of BDs using their photometric parameters as a decision rule. For example, Skrzypek et al. (2016) successfully used the data from three major surveys: Sloan Digital Sky Survey (SDSS), UKIRT Infrared Deep Sky Survey (UKIDSS) and Wide-Field Infrared Explorer (WISE). They used the constraints ( Y J ) Vega > 0.8 and J < 17.5 as a decision rule. It was possible to detect about 1,300 BDs in an area of 3,000 square degrees (about 7.5% of the celestial sphere). The constraints on photometric parameters were also developed by Carnero Rosell et al. (2019). In this case, the following constraints were used: ( i z ) > 1.2 , ( z Y ) > 0.15 , ( Y A B J Vega ) > 1.6 , and z < 22 . The limitation on the z magnitude was made to achieve the completeness of the data. Almost 12,000 BDs were selected in an area of 2,400 square degrees (about 5.8% of the celestial sphere).

These works mostly deal with distant BDs, so these rules are developed under such limitation. Distant BDs are less favorable in terms of model testing since it requires absolute magnitudes, which are difficult to calculate for distant objects due to the possible low accuracy (or absence) of parallaxes and poor extinction knowledge. Moreover, the nearest BDs could serve as targets for spectroscopic observations. In addition, following Vos et al. (2019), we distinguish three groups of BDs and treat them independently, which has not been done previously.

Here, we report on developing a method of cross-matching high-proper motion nearest BDs with large photometric surveys based on photometric rules that are applicable to each BD object. These rules can help one to distinguish the right match for a BD in a particular survey, even if its position is distant.

2 Sky survey overview

This work will be dealing with four major sky surveys (two micron all-sky survey (2MASS) WISE (AllWISE), early data release 3 (Gaia EDR3) and SDSS). Here, we give an overview of these surveys. The main information on the surveys is summarized in Table 1, and response curves of their photometric systems are shown in Figure 1.

Table 1

Summary of main parameters of sky surveys

Survey N , 1 0 6 Sky coverage Photometric bands and limiting magnitude
SDSS DR12 469 35% u = 2 2 m , g = 22 . 2 m , r = 22 . 2 m
i = 21 . 3 m , z = 20 . 5 m
Gaia EDR3 1,811 All sky G = 2 0 m (low galactic latitudes)
G = 2 2 m (high galactic latitudes)
WISE 564 All sky W 1 = 16 . 6 m , W 2 = 15 . 6 m
W 3 = 11 . 3 m , W 4 = 8 . 0 m
2MASS > 500 All sky J = 15 . 8 m , H = 15 . 1 m , K s = 14 . 3 m

N is the number of objects in millions. Limiting magnitudes for SDSS are the levels of 95% completeness for point sources, for Gaia EDR3 are the levels of 99% completeness for point sources, for WISE are given for the point-sources above the SNR > 5 , and for 2MASS are for the point sources above SNR > 10 .

Figure 1 
               Response curves of photometric systems.
Figure 1

Response curves of photometric systems.

2MASS is a whole-sky infrared survey, in which observations were carried out from 1997 to 2001 using two 1.3 m telescopes in the Northern and Southern hemispheres (Cutri et al. 2003, Skrutskie et al. 2006). The sky was observed in three near-infrared photometric bands ( J , H , and K s ). The response curves – the dependence of the filter transmittance on the wavelength – for these filters and filters of other systems are shown in Figure 1. The results of the mission are the Point Source Catalog consisting of over 500 million stars and galaxies, the Extended Source Catalog consisting of 1.6 million resolved galaxies, the Large Galaxy Atlas consisting of 600 nearby galaxies and globular clusters, and the All-Sky Quicklook and Atlas images providing full coverage of the infrared sky. For additional information, see https://irsa.ipac.caltech.edu/Missions/2mass.html website.

SDSS is one of the first modern sky surveys, in which observations are carried out using a 2.5 m wide-angle optical telescope equipped with a set of imagers and spectrographs (Alam et al. 2015). The telescope is located at Apache Point Observatory in New Mexico. During the existence of the project (it still operates), it had many scientific goals: from the creation of the detailed three-dimensional map of the Milky Way to an exoplanet search. The data collection began in 2000 and has become available to the public in the form of data releases (usually referred to as DR). DR12 contains information on 470 million objects, it includes all SDSS data taken through 14 July 2014. Data contain images of the sky in five filters named u , g , r , i , and z . Table 1 shows information on limiting magnitudes. Additional information on the SDSS project can be found at https://www.sdss.org/dr12/ website.

Gaia is a space-born survey (Gaia Collaboration et al. 2018), and the mission of the observatory is to measure precisely the positions of stars both in the Milky Way and other parts of the Local Group, i.e., to make a three-dimensional map. The astrometric measurements are complemented by spectral and photometric measurements of all objects. The spacecraft is located at the second Lagrange point. This point lies on the line through the Sun and the Earth, beyond the last one, and is about 1.5 million km far from the Earth. Gaia consists of three scientific instruments (or three-in-one): tools for astrometry and photometry, and the spectrometer used for measurements of objects, radial velocities. Additional information can be found at the project’s web page: https://www.cosmos.esa.int/web/gaia/.

Another space observatory, WISE, provided All-Sky Survey in infrared (Wright et al. 2010, Cutri et al. 2012). The telescope was launched in December 2009. The All-Sky WISE data became available in the form of DR by March 2012, which includes all the data obtained from December to August 2010. WISE explores the sky in the following four filters: W1 ( 3.4 μ m ), W2 ( 4.6 μ m ), W3 ( 12 μ m ), and W4 ( 22 μ m ). The highlights of the discoveries include discovering the most luminous galaxy in the universe, revealing millions of black holes, and discovering new types of stars.

3 Identification of three families of L, T, and Y BDs

We took the list compiled by Kirkpatrick et al. (2021) as a source of L, T, and Y BDs with well-studied photometry obtained from 2MASS and WISE, as well as astrometric data such as parallax, proper motions, coordinates, and epoch, on which these coordinates were recorded. The list (hereafter K2021) contains this and other ( T eff , spectral type) information on 496 objects. Except for spectral types > T8 and T eff < 600 K , K2021 is statistically complete to the 20 pc limit.

According to Vos et al. (2019), one can expect to find three types of objects that manifest themselves differently in photometry, namely, L dwarfs, L / T -transition dwarfs, and T and cooler dwarfs.

As we move toward lower temperatures on the Hertzsprung–Russell diagram, the color of BDs becomes significantly bluer (e.g., in J K s color, see Figure 2, right), breaking the monotonous color change. At the same time, they become brighter ( H color, Figure 3, top right), also breaking the monotonous trend of brightness variation with decreasing temperature. This phenomenon is apparently associated with a change in the properties of the atmosphere of BDs. Some atmospheric models suggest that this is due to dust clouds descending through the photosphere, resulting in a rapid and dramatic change in color characteristics (Marley and Robinson 2015, Charnay et al. 2018). There are also models that relate to chemical instability in the atmosphere (Tremblin et al. 2019). Another suggestion is the so-called patchy cloud models (Artigau et al. 2009). They are motivated by the observed variability of L- and T-type BDs.

Figure 2 
               Photometric, color, and spectral characteristics of three families, objects. Bright objects are indicated in blue, transit in green, and faint in red.
Figure 2

Photometric, color, and spectral characteristics of three families, objects. Bright objects are indicated in blue, transit in green, and faint in red.

Figure 3 
               
                  
                     
                        
                        
                           
                              
                                 K
                              
                              
                                 s
                              
                           
                           −
                           W
                           1
                        
                        {K}_{s}-W1
                     
                   color index vs proper motion in arcsec per year (top left). 
                     
                        
                        
                           H
                           −
                           W
                           1
                        
                        H-W1
                     
                   – 
                     
                        
                        
                           H
                        
                        H
                     
                   diagram (top right). 
                     
                        
                        
                           
                              
                                 K
                              
                              
                                 s
                              
                           
                           −
                           W
                           1
                        
                        {K}_{s}-W1
                     
                   – 
                     
                        
                        
                           W
                           1
                           −
                           W
                           2
                        
                        W1-W2
                     
                   diagram (bottom left). 
                     
                        
                        
                           W
                           1
                           −
                           W
                           2
                        
                        W1-W2
                     
                   – 
                     
                        
                        
                           W
                           1
                        
                        W1
                     
                   diagram (bottom right). Bright BDs are indicated in blue, transit in green, and faint in red.
Figure 3

K s W 1 color index vs proper motion in arcsec per year (top left). H W 1 H diagram (top right). K s W 1 W 1 W 2 diagram (bottom left). W 1 W 2 W 1 diagram (bottom right). Bright BDs are indicated in blue, transit in green, and faint in red.

That is why we divide our list into three groups before the crossmatching. We divide all objects into groups according to the S p A d parameter of K2021. S p A d is a numerical code of spectral classification, chosen by Kirkpatrick et al. (2021). S p A d = 0 9 is for L 0 L 9 , and S p A d = 10 19 for T 0 T 9 , S p A d = 20 24 for Y 0 Y 4 . We call objects with S p A d < 9 bright, objects with 9 S p A d < 14 transit and objects with S p A d 14 faint. Their photometric, spectral and color properties are presented in Figure 2. We crossmatched different groups separately, so every group will have its own rules of crossmatching.

4 Comparing 2MASS and AllWISE photometry

K2021 contains information in AllWISE photometric bands and J, H, and K s of 2MASS photometric bands, so we start with exploring corresponding color indices of the objects.

The top left plot of Figure 3 shows a K s W 1 color to the total proper motion μ tot in arcsec per year. One can see that our objects demonstrate a variety in this color index, especially fainter red objects (up to 5 mag spread). In the same Figure 3, we present different color–magnitude and color–color diagrams. All of these color indices demonstrate a fairly large scatter (up to several magnitudes), which means that these color indices cannot be used as indicators of successful crossmatching.

As seen in the top left plot of Figure 3, the proper motions can be as high as 5 arcsec/year, which is a potential problem as the 2MASS/WISE epoch difference is about 10 years, which results in a potential 50-arsec gap between the same objects.

5 Crossmatching with SDSS and Gaia EDR3

We have the J2000 coordinates for each object at epoch MJD 57170.0 from K2021. As our objects have high proper motions, according to Kirkpatrick et al. (2021), the essential thing to do before crossmatching is to convert the coordinates to the J2000 epoch. We transformed the coordinates via the following simple formula:

RA J 2000 = RA pm RA cos ( Dec ) ( 2457170.5 2451545.0 ) Dec J 200 = Dec pm Dec ( 2457170.5 2451545.0 ) ,

where pm RA and pm Dec are proper motions in degrees per day.

Although the observations presented in SDSS DR12 were carried out through 2014, SDSS provided proper motions for some objects, and their positions were adjusted to J2000 epoch where possible. The same applies even more to Gaia, since Gaia measures proper motions for most observed objects with high accuracy, and all positions for Gaia EDR3 objects were given for the J2000.0 epoch, which is available in VizieR.

The method for obtaining the rules is discussed further. We search for our objects with new J2000 coordinates in a particular survey with a crossmatching radius of 2 arcsec. Taking a larger radius seems to be dangerous, since it is more likely that random objects not related to our BDs will fall into this radius and it would be harder to distinguish the correct matches.

5.1 SDSS

The crossmatching with SDSS with these parameters gave us 71 objects, from which we eliminated nine poor-quality measurements, leaving only those with the Q flag equals to 3 (good quality observations). The results are presented in Figure 4, x -axis is an angular distance between our coordinates and the coordinates of the match in SDSS DR12. Objects in all three groups seem to have ( i z ) color index in the particular range, except for five ones in the bottom of the plot, which are indicated by crosses.

Figure 4 
                  Results of the crossmatching the K2021 with SDSS DR12 with 2 arcsec crossmatching radius. Red squares are faint objects, green circles are transit objects, blue diamonds are bright objects. Crosses indicate suspicious matches.
Figure 4

Results of the crossmatching the K2021 with SDSS DR12 with 2 arcsec crossmatching radius. Red squares are faint objects, green circles are transit objects, blue diamonds are bright objects. Crosses indicate suspicious matches.

We plotted the same objects on a magnitude–magnitude diagram with i on one axis and z on the other (Figure 5, left). Crossed objects are significantly bluer than the objects of the same group or even bluer than BD in general. Figure 5 right is an image of a counterpart for WISEPC J234446.25 + 103415.8 in r and i SDSS bands and Centre de Donnees astronomiques de Strasbourg (CDS) color composition image. It is, as seen, also much bluer than typical BD. The same can be said about other faint crossed objects.

Figure 5 
                  Results of the crossmatching on magnitude–magnitude diagram (left). Image of the match found for WISEPC J234446.25 + 103415.8 in SDSS DR12 (right) in filters 
                        
                           
                           
                              r
                           
                           r
                        
                      and 
                        
                           
                           
                              i
                           
                           i
                        
                      and CDS color composition image.
Figure 5

Results of the crossmatching on magnitude–magnitude diagram (left). Image of the match found for WISEPC J234446.25 + 103415.8 in SDSS DR12 (right) in filters r and i and CDS color composition image.

Interesting case is bright crossed object (in blue). Apparently, the identification is correct. However, there is a bright infrared source near that object: a variable star of Mira Cet type Z Peg, so we think that the results may be somehow contaminated, although there is no indication of poor-quality observation in the SDSS data. Despite calling “blue” BD objects bright, they are still much fainter than some other sources, so we decided to throw this match out.

The corridors into which the color indices of the remaining objects fall can be described by mean and standard deviation and are presented in Table 2. When constructing the color indices for Table 2, we chose photometric bands close in position on the wavelength scale. Here we assume that the color indices do not correlate with other parameters (magnitude, parallax, proper motion, etc.), which, given the limitations of the test set, seems to be reasonable. Due to insufficient statistics, we are forced to assume that the distributions over color indices are single-mode and symmetrical (Figures 4 and 6), and thus we can use the terms “mean value” and “standard deviation.”

Table 2

Constraints on color indices between SDSS, 2MASS, and Gaia

i z z J z H R P J
Bright 2.2 ± 0.4 2.64 ± 0.15 3.58 ± 0.26 3.43 ± 0.26
Transit 3.9 ± 0.74 2.94 ± 0.17 3.88 ± 0.17 3.86 ± 0.2
Faint 4.1 ± 0.7 3.32 ± 0.2 3.42 ± 0.26 4.0 ± 0.7
Figure 6 
                  (
                        
                           
                           
                              z
                              −
                              J
                           
                           z-J
                        
                     ) Color index to the distance between the object and the match. Crosses indicate suspicious matches.
Figure 6

( z J ) Color index to the distance between the object and the match. Crosses indicate suspicious matches.

We have also investigated ( z J ) color index in the same fashion. The results are presented in Figure 6. The suspicious matches from Figure 4 are again indicated with crosses. As one can see, two of them stand out again, although the rest lie within reasonable limits. However, we throw them all out again. Another object in the left bottom corner is a spurious detection as well. All of the remaining lies pretty well in corridors, and their properties are summarized in Table 2, as well as the properties of the ranges for ( z H ) color indices.

5.2 Gaia EDR3

From crossmatching with Gaia EDR3, we have obtained 135 matches, of which 112 belong to the bright family, 10 – to the transit family, and 13 to the faint family. Apparently, transit and faint objects are too faint for Gaia. Unlike the crossmatching with SDSS DR12, not all of the objects, found in Gaia, has a value of J magnitude in K2021. As a result, we have only seven faint objects that have both 2MASS and Gaia photometry, which is not sufficient for any statistics. However, for now, we keep our approach unchanged. The results of crossmatching are presented in Figure 7

Figure 7 
                  (
                        
                           
                           
                              R
                              P
                              −
                              J
                           
                           RP-J
                        
                     ) Color index to the distance between the object and the match. Crosses indicate suspicious matches.
Figure 7

( R P J ) Color index to the distance between the object and the match. Crosses indicate suspicious matches.

We have several clear outliers for bright objects, indicated with the crosses in Figure 8. Interesting case is an object on the right side on the plot. It is in fact a binary BD system: Luhman 16A and Luhman 16B, which was probably unresolved by 2MASS in K2021. The top left object is most likely a correct match, with the reason for such a sharp difference left for further study in subsequent works.

Figure 8 
                  Magnitude–magnitude diagram.
Figure 8

Magnitude–magnitude diagram.

The mean and standard deviations are presented in Table 2. Note that transit and especially faint objects are rare, so these values are less reliable.

We have also investigated ( B P R P ) color index of Gaia for matched objects. Yet, colors of different objects are diverse and do not allow one to obtain any rules for crossmatching.

6 Conclusion

This work is dedicated to developing a method of cross-matching of high-proper motion L, T, and Y BDs with different photometric surveys. To achieve that goal, we investigated different color indices in a number of sky surveys: 2MASS, WISE, SDSS, and Gaia for BDs. We have determined the color indices that can be used to search for these objects in large photometric surveys, and we have shown that these color indices do not depend on other parameters (magnitude, parallax, proper motion, etc.). We have also pointed out the color indices that are inappropriate for this purpose. We developed the rules that can help one to find correct matches for BDs in the process of cross-matching with a large radius matching. The rules are different for bright, transit, and faint BDs (Table 2). We are aware that other objects (e.g., red dwarfs) can also meet these criteria. However, we hope that the inclusion of other photometric surveys in the study will improve the situation: the more color indices (and rules for them), the fewer extraneous objects in the final sample.

We also made preliminary estimates of the radius matching to search for objects in the modern photometric surveys, given their proximity to the Sun and, consequently, their high proper motion. We plan to continue this work by implementing these rules to the large-scale crossmatching of the surveys without the knowledge of proper motions aiming to find new candidates for BD. According to our estimates, over 10 years (the average epoch difference between 2MASS and SDSS), such objects can fly away for 50 arcsec or even more, which makes them difficult to find by classical crossmatching methods.

We also plan to work on refining the borders between the three families of BD in order to clarify our rules, since the separation that we have applied may not be entirely correct.

Acknowledgments

We are grateful to our reviewer, whose constructive comments greatly helped us to improve the article.

  1. Funding information: This study was funded by the Ministry of Science and Higher Education of the Russian Federation, according to the research project 13.2251.21.0177.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and have approved its submission.

  3. Conflict of interest: The authors state that there is no conflict of interest.

  4. Data availability statement: The data underlying this article will be shared on reasonable request to the corresponding author.

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Received: 2022-12-13
Revised: 2023-01-11
Accepted: 2023-01-22
Published Online: 2023-02-16

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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