Exploring the planetary-mass population in the Upper Scorpius association
MMNRAS , 1–14 (2015) Preprint 1 March 2021 Compiled using MNRAS L A TEX style file v3.0
Exploring the planetary-mass population in the Upper Scorpiusassociation ★ N. Lodieu , † , N. C. Hambly , N. J. G. Cross Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, E-38200 La Laguna, Tenerife, Spain Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain Scottish Universities Physics Alliance (SUPA), Institute for Astronomy, School of Physics and Astronomy, University of Edinburgh,Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
Accepted 1 March 2021. Received 1 March 2021; in original form 1 March 2021
ABSTRACT
We aim at identifying very low-mass isolated planetary-mass member candidates in the nearest OB association to the Sun, UpperScorpius (145 pc; 5–10 Myr), to constrain the form and shape of the luminosity function and mass spectrum in this regime. Weconducted a deep multi-band ( 𝑌 = 21.2, 𝐽 = 20.5, 𝑍 = 22.0 mag) photometric survey of six square degrees in the central region ofUpper Scorpius. We extend the current sequence of astrometric and spectroscopic members by about two magnitudes in 𝑌 andone magnitude in 𝐽 , reaching potentially T-type free-floating members in the association with predicted masses below 5 Jupitermasses, well into the planetary-mass regime. We extracted a sample of 57 candidates in this area and present infrared spectroscopyconfirming two of them as young L-type members with characteristic spectral features of 10 Myr-old brown dwarfs. Among the57 candidates, we highlight 10 new candidates fainter than the coolest members previously confirmed spectroscopically. We donot see any obvious sign of decrease in the mass spectrum of the association, suggesting that star processes can form substellarobjects with masses down to 4–5 Jupiter masses. Key words:
Stars: low-mass stars and brown dwarfs — techniques: photometric, spectroscopic — surveys — stars: luminosityfunction, mass function
The existence of brown dwarfs and exoplanets is now well estab-lished with several hundreds of examples discovered over the past25 years (Mayor & Queloz 1995; Rebolo et al. 1995; Nakajimaet al. 1995). Large-scale surveys for field ultracool dwarfs and deeppencil-beam surveys of specific regions have now identified substel-lar objects down to a few Jupiter masses with properties differingdepending on their age, environment or metallicity. The shape ofthe substellar present-day mass function (Salpeter 1955; Miller &Scalo 1979; Scalo 1986) in the solar neighbourhood seems to indi-cate a power law index 𝛼 of 0.6, where dN/dM ∝ M − 𝛼 accountingfor all members of the 20 pc sample (Kirkpatrick et al. 2019). Thisslope is consistent with the substellar shape of the mass functions instar-forming regions (review by Bastian et al. 2010), nearby movinggroups (Liu et al. 2013; Gagné et al. 2015b; Faherty et al. 2016), andyoung clusters (review by Luhman 2012). How massive can the leastmassive fragment that star formation mechanisms can form (also re-ferred to as the limit of fragmentation), remains an open question.Early estimates suggested 7 Jupiter masses (hereafter M Jup ) (Low &Lynden-Bell 1976; Rees 1976; Silk 1977) but this limit may changewhen including rotation or magnetic fields in the simulations (Boss ★ Based on observations collected at the European Organisation for As-tronomical Research in the Southern Hemisphere under ESO programmes089-C.0102(ABC), 097.C-0781(A), and 0101.C-0565. † E-mail: [email protected]
Jup .Upper Scorpius (USco) is part of the nearest OB association to theSun, Scorpius Centaurus. The region is located at 145 pc from theSun (de Bruijne et al. 1997) and has an age of 5–10 Myr (Preibisch& Zinnecker 1999; Preibisch et al. 2001; Pecaut et al. 2012; Songet al. 2012; David et al. 2019). USco members show a significantmean proper motion compared to stars along its line of sight (meanvalue of −
11 and −
25 mas/yr in right ascension and declination,respectively; de Bruijne et al. 1997; de Zeeuw et al. 1999). Thehigh-mass population of USco has been explored in X-rays (Walteret al. 1994; Kunkel 1999; Preibisch et al. 1998), astrometrically(de Bruijne et al. 1997; de Zeeuw et al. 1999; Cook et al. 2017;Luhman & Esplin 2020a) in the optical (Ardila et al. 2000; Martínet al. 2004; Slesnick et al. 2006, 2008), and in the infrared (Lodieuet al. 2006, 2007a; Kraus et al. 2008; Béjar et al. 2008; Lafrenièreet al. 2010; Dawson et al. 2011; Lodieu et al. 2011a; Lafrenièreet al. 2011; Dawson et al. 2013; Lodieu 2013; Lafrenière et al. 2014;Dawson et al. 2014; Best et al. 2017). Low-mass stars and browndwarfs in USco have been subject to numerous studied over the pastyears thanks to the arrival of wide and deep surveys (Preibisch et al.2001; Preibisch & Zinnecker 2002; Lodieu et al. 2007a, 2011b; PeñaRamírez et al. 2016), and more recently the
𝐺𝑎𝑖𝑎 mission (GaiaCollaboration et al. 2018), yielding a relatively well-defined mass © a r X i v : . [ a s t r o - ph . S R ] F e b N. Lodieu et al. function below the stellar/substellar limit in this region (Luhman &Esplin 2020a). The Kepler K2 mission (Borucki et al. 2010; Lissaueret al. 2014; Batalha 2014) has revealed the first eclipsing binaries inUSco over a wide range of masses, giving the first independent massand radius determinations at 5–10 Myr (Alonso et al. 2015; Krauset al. 2015; Lodieu et al. 2015; David et al. 2016b, 2019) as well asthe first Neptune-size planet orbiting a M3 star (Mann et al. 2016;David et al. 2016a).Lodieu et al. (2018a) recently identified the first L dwarf sequencein USco with optical and infrared spectroscopic characterisation.These authors identified L-type candidates in a deep
𝑍𝑌 𝐽 survey(Lodieu et al. 2013b) conducted with the Visible Infrared SurveyTelescope (VISTA; Emerson 2001; Dalton et al. 2006). In this paperwe release a new, 𝑌 -band survey 2 magnitudes deeper over half of thearea compared to the former survey to look for isolated planetary-mass candidates with ages of 5–10 Myr and investigate the shape ofthe luminosity and mass function below the deuterium-burning limit.In Section 2 we present the deep infrared observations obtained withEuropean Southern Observatory (ESO) VISTA/VIRCAM located inParanal (Chile) and the data reduction methods. In Section 3 wedetail the Sloan 𝑧 -band survey conducted with the Hyper Suprime-Cam camera on Subaru in Mauna Kea (Hawaii, USA). In Section 4 wedescribe the selection of potential planetary-mass member candidatesof the USco association. In Section 6 we discuss the nature of ourcandidates and the implications on the shape of the IMF and thetheory of the fragmentation limit. This study makes use of two different but complementary VISTAsurveys described below. VISTA is a 4-m telescope (Emerson 2001;Emerson et al. 2004) based in Paranal (Chile) and equipped with theVISTA InfraRed CAMera (VIRCAM; Dalton et al. 2006). VIRCAMis fitted with 16 infrared detectors with pixels of 0.339 arcsec offeringa field of view of 1.65 square degrees.
On the one hand, we re-processed the deep VISTA
𝑍𝑌 𝐽 survey de-scribed in Lodieu et al. (2013b) to extract PSF photometry insteadof aperture photometry for optimal completeness and best accuracyfor faint point sources in these relatively crowded fields. To sum-marise, these surveys cover 13.5 square degrees towards the centralregion of USco. The observations took place between April and May2012 down to 100% completeness limits of 𝑍 = 22.0 mag, 𝑌 = 21.2mag, and 𝐽 = 20.5 mag, respectively. We refer the reader to Lodieuet al. (2013b) for more details. We will refer to that survey as the‘first epoch’ throughout the paper. This work identified tens of browndwarf and planetary-mass members, 12 of them being confirmedspectroscopically as 5–10 Myr-old L1–L7 members (Lodieu et al.2018a) with an independent classification by Luhman et al. (2018).On the other hand, we conducted a deeper 𝑌 -band survey withVISTA/VIRCAM to be sensitive to fainter and cooler USco mem-bers because the first epoch was limited in depth by the 𝑌 and 𝑍 filters. We observed four VISTA/VIRCam tiles in the 𝑌 -band filter(centered at 1.02 ± 𝑌 = 22.6mag over 6.6 square degrees. Due to the differences in the tilingconfigurations automatically processed by the ESO survey definitiontool, the common area between the two VISTA survey is about sixsquare degrees (Fig. 1). VISTA Science Archive (VSA; Cross et al. 2012) standard pipelineproducts consist of instrumentally corrected images combined intostacks and source catalogue extraction with fixed apertures from thoseimages. While the fixed–aperture source extraction (Irwin et al. 2004)is optimised for faint point sources, the detection and deblending offaint stars, in particular those near to much brighter objects, is notoptimal. Our deepest stacked images are rather more crowded thanthe shallow and wide survey images for which the aperture sourceextraction has been optimised, so we employed Point Spread Function(PSF) fitted photometry in creating our source detection lists fromthe VSA image products (see for example Mauro et al. 2013).We employed DAOPHOT (Stetson 1987) in a recent incarnationrunning within the IRAF/PyRAF environment (Tody 1986, 1993;Davis 1999; Greenfield & White 2006; Science Software Branchat STScI 2012). We followed standard best–practice in detectingsources, defining the PSF and then using that PSF to fit all detectedobjects, simultaneously within small groups, to extract source posi-tions and fluxes. Matched–filter source detection at a 2 . 𝜎 thresholdwas run on all the images using the DAOFIND task. A selectionof 100 PSF stars was made for each image automatically from theDAOFIND output, where every PSF star was required to have a mag-nitude between 3.0 and 3.5 above the detection limit (i.e. intermediatein brightness for good signal–to–noise but not so bright as to be af-fected by non–linearity or saturation), and having no other detectedsource within two PSF fitting radii. Additionally we required ourPSF stars to have values of the DAOFIND shape parameters SHARP,SROUND and GROUND within 1 𝜎 of their median values in orderto restrict the selection to faint, point–like images in every case. ThePSF model in DAOPHOT consists of an underlying analytical func-tion with additional residual corrections (Stetson 1987). In order tochoose the most appropriate analytical model amongst the availableoptions, we made trial fits and compared the residual profile scatterfor all. We finally settled on the ‘penny2’ option (a Gaussian core withLorentzian wings, both of which are elliptical, and tilted arbitrarilyand independently to the image coordinate axes). In order to accountfor position–dependent variation in the PSF we selected ‘varorder’equal to 2 for quadratically varying terms in the residual correctionlook–up tables of the combined PSF model. A default PSF modelradius of 11 pixels was employed during PSF modelling, while aPSF fitting radius of twice the full-width-half-maximum (FWHM)measured in each image was employed at the PSF photometry stagewhen photometring all detected sources.We matched the 𝑌 and 𝐽 catalogues within a conservative radiusof 3 arcsec, yielding a total of 887,175 sources. We applied twoquality criteria on the sharpness (between − 𝜒 (lessthan 3.0) parameters in both filters that returned 617,715 objects,which will be our input catalogue in the rest of our analysis. We notethat the number of sources with pairing radii larger than 1 arcsecis relative constant. We count 7106 pairs beyond 1 arcsec, resultingin a contamination less than 1.2%. We show the resulting ( 𝑌 − 𝐽 , 𝐽 )colour-magnitude diagram in the left-hand side of Fig. 2. In the right-hand side of Fig. 2, we plot the same diagram and overplot knownUSco members from the literature and our photometric candidates. MNRAS , 1–14 (2015) xploring the IPMO population in USco Table 1.
Logs of the VISTA/VIRCAM 𝑌 -band observations. We list the name of the tiles with their coordinates in sexagesimal format, the range of date + timeof observations, the total on-source exposure time, the median seeing, mean ellipticity, and airmass at the time of observations. We give the average parametersfor each stacked tile made of seven individual tiles combined together. Tile R.A. Dec Date_min Date_max ExpT Seeing Ell Airmasshh:mm:ss.ss ◦ : (cid:48) : (cid:48)(cid:48) yyyy-mm-dd yyyy-mm-dd seconds arcsectile112 243.2118900 − − − − Figure 1.
Location of the four deep 𝑌 -band tiles (light grey) on top of the ninefirst epoch observations conducted in 𝑍𝑌 𝐽 (grey) with VISTA (Lodieu et al.2013b). Overplotted is the coverage of the deep Subaru HSC survey (lightrose). Known members from Luhman & Mamajek (2012) and Lodieu et al.(2018a) are shown as blue open squares and triangles, respectively. The newcandidates selected in this work are highlighted as black and orange symbols.
We cross-matched the
𝑌, 𝐽 catalogue with photometry from ourprevious VISTA 𝑍 -band as well as 𝐻 + 𝐾 from the UKIRT InfraredDeep Sky Survey (UKIDSS; Lawrence et al. 2007) Galactic ClustersSurvey (GCS) Data Release 9 (DR9), the AllWISE survey (Cutri et al.2013; Cutri & et al. 2014), and the Panoramic Survey Telescopeand Rapid Response System catalogue (Pan-STARRS; Chamberset al. 2016) with a pairing radius of 3 arcsec in each case. Wewill make the full catalogue available to the community via Vizierat the Centre de Données de Strasbourg. The full table containsthe coordinates in J2000, the magnitudes in each filter with theirassociated uncertainties, the sharpness and 𝜒 parameters given by daophot and allstar in IRAF, the proper motion in mas, andthe cross-match with other catalogues like the UKIDSS GCS DR9,AllWISE, and PanStarrs DR1. The Hyper Suprime-Cam (HSC) is a very large charge–coupled de-tector (CCD) mosaic camera mounted at prime focus of the Subaru8.2-m Telescope (Iye et al. 2004) in Mauna Kea, Hawaii. The HSCuses 104 science charge-coupled devices sensitive to optical wave-lengths, offering a 1.5-degree field-of-view in diameter with a pixelsize of 0.17 arcsec (Miyazaki et al. 2018; Komiyama et al. 2018;Kawanomoto et al. 2018; Furusawa et al. 2018). The auto-guidingis ensured with four CCDs while the focus is monitored with eightCCDs. We employed the Sloan 𝑧 -band filter centered at 8921.66 Åwith a width of 792.99 Å to be as sensitive as possible to the coolestmembers of the association, with the goal of improving on the depthof the deep 𝑍 survey (Section 2.1).The observations were collected in service mode on 25 May 2020between UT = 10h58 and 12h40 as part of programme S20A-QN098(PI Lodieu). The airmass of USco from Mauna Kea was between 1.4and 1.8. The seeing was better than 0.8 arcsec. The transparency washigher than 90% and the sky was dark with no moon. We collectedfour blocks of six integrations of 252 seconds, yielding a total expo-sure of 5040s on a single 1.5-degree field centered at RA = 16h13m,dec = 22 ◦ (cid:48) (Fig. 1). A standard data reduction procedure was employed in processing theHSC via the LSST pipeline stack (Jenness & LSST Data Manage-ment Team 2017). Instrumental correction consisted of bias pres-can/overscan, dark, illumination and flat corrections along with adefault brighter–fatter kernel correction (Bosch et al. 2018). Single–visit processing to create accurate astrometric and photometric cal-ibration at the individual CCD level was performed with respectto the PanSTARRS DR1 catalogue (Chambers et al. 2016). The 20individual frames from each of the 104 CCDs available within thefield visit where warped and stacked in patches to create a deep stackimage from which sources were extracted and measured for point-spread function (PSF) fitted photometry, again using the standardLSST routines. The HSC– 𝑍 band photometry, by default on an ABmagnitude scale, was transformed to the VIRCam Vega system usinga correction 𝑧 Vega = 𝑧 AB − .
521 (Cross et al. 2012). Finally, asmall ± .
05 linear colour correction was applied to the photometryas a function of 𝑧 − 𝐽 colour to put the HSC– 𝑍 measurements on thenatural VIRCam 𝑍 photometric system.We extracted a total 853,676 sources with a completeness limit of 𝑍 = 24.3 mag, reduced to 379,279 objects when removing saturatedsources (typically brighter than 17.5 mag), and extended sources as MNRAS , 1–14 (2015)
N. Lodieu et al. well as those with poor PSF fitting. We cross-matched the full deep
𝑍𝑌 𝐽
VISTA catalogue with the good-quality sources in the HSC cat-alogue and found 162,735 objects in common. We will make this fullVISTA + Subaru catalogue of identifiers, coordinates, 𝑍 photometrywith its associated error, classification (point source or extended),and flag available to the community via Vizier. The goal of this section is to focus on member candidates fainter thanthe coolest L dwarfs identified in our original
𝑍𝑌 𝐽 survey (Lodieuet al. 2013b) and recently confirmed spectroscopically (Lodieu et al.2018a) by combining the deep 𝑌 -band second epoch survey with thefirst epoch 𝑍𝑌 𝐽 survey. To set the scene, the two L7 spectroscopicmembers have 𝑌 = 20.8–21.0 mag, 𝐽 ∼ 𝑍 magnitudesbeyond the 100% completeness limit of the first epoch ( 𝑍 = 22 mag).We start by reviewing our current knowledge on the photometricproperties of field and young L and T dwarfs before jumping into theselection of USco brown dwarfs and isolated planetary-mass objects.We emphasise that studies of isolated planetary-mass objects ofT-type in clusters and star-forming regions younger than 20 Myr cur-rently are rather limited. Most objects are candidates whose mem-bership remains under debate or that need to be confirmed throughspectroscopy and/or astrometry (e.g. Burgess et al. 2009; Haisch et al.2010; Peña Ramírez et al. 2011; Barsony et al. 2012; Spezzi et al.2012; Scholz et al. 2012; Lodieu et al. 2013a; Chiang & Chen 2015;Peña Ramírez et al. 2015). We looked at the colours and absolute magnitudes of field L and Tdwarfs (Dupuy & Liu 2012) and located them in several diagrams(see figures in this paper) to guide the identification of their youngercounterparts. We draw the following conclusions, keeping in mindthat our current poor knowledge of the spectral energy distributionof T-type objects at young ages. • The 𝑌 − 𝐽 colours of field L/T dwarfs are approximately constantaround 1 mag, going from 1.2 mag for L6 to 1.1 mag to T8 but withvalues slightly below 1.0 mag for T2 and T3 types (Dupuy & Liu2012). From the synthetic colours of L/T dwarfs (Hewett et al. 2006),a kink towards the blue is observed in the 𝑌 − 𝐽 colours for transitionsources followed by a relatively constant value from mid-T dwarfs(right-hand side panel in Fig. 2). • The 𝑍 − 𝐽 vs 𝑌 − 𝐽 diagram indicates that L/T transition objectshave 𝑍 − 𝐽 colours as blue as 2.5 mag (Hewett et al. 2006) witha larger dispersion for L/T transition dwarfs and an average 𝑍 − 𝐽 colour that keeps being redder with later spectral types (Fig. 3). • The 𝑍 − 𝐾 colours of L/T transition objects show a similardispersion as in 𝑍 − 𝐽 while their 𝐽 − 𝐾 colours become bluer fromT0 ( 𝐽 − 𝐾 = 1.4 mag) to T1 ( 𝐽 − 𝐾 = 1.13 mag) as seen in the rightpanel in Fig. 4. • The absolute 𝑌 magnitude of a T4 is as bright as a L7 but slightlyfainter than a L6. • A old field T4 dwarf has SDSS 𝑧 ∼ 𝑌, 𝐽 = 21.13, 20.13 mag (Dupuy & Liu 2012)but USco members tend to be 1 mag brighter than field objects.
We examined the colours of five red young L7 dwarfs within50 pc with ages less than 20 Myr (determined via membershipof young moving groups) similar to USco recently published inthe literature (e.g. Looper et al. 2007; Gauza et al. 2015; Gagnéet al. 2015a; Filippazzo et al. 2015; Faherty et al. 2016; Liuet al. 2016; Schneider et al. 2016). We also compiled three Tdwarfs with older ages but well-constrained ages based on theirhigh-probability membership of the AB Dor (GUPSc b (T3.5),SDSS J11101001 + + 𝑍 magnitudes published for these sources,which appears as a key filter in our study. We also added to the dia-grams three L/T transition member candidates (blue squares) of thePleiades (Zapatero Osorio et al. 2014b) at an age of 125 Myr (Staufferet al. 1998) identified in a deep photometric survey with astrometricinformation (Zapatero Osorio et al. 2014a). We plot all these youngL/T transition sources with 𝑌, 𝐽, 𝑤 , 𝑤 𝑌 − 𝐽 colours butredder colours in other combinations of filters that may come fromdifferences in the optical thickness of the dust cloud deck (Maroccoet al. 2014). As a first step, we carefully looked at the ( 𝑌 − 𝐽 , 𝑌 ) diagram in theleft-hand side panel in Fig. 2. We suggest that the USco sequence isvisible, running from around ( 𝑌 − 𝐽 , 𝑌 ) = (0.75, 15.0) to (1.45, 20.0) to(2.6, 22.5), with the reddest candidate at around (3.1, 22.7). The firstpart of the USco sequence down to 𝑌 = 20 mag is supported by knownmembers while the bottom part is based on the morphology of thecolour-magnitude diagram. To confirm this hypothesis, we overploton this diagram known USco members from Luhman & Mamajek(2012) and spectroscopic L dwarf members (Lodieu et al. 2018a),displayed as open blue squares and triangles in Fig. 2, respectively.Hence, we designed a series of selection lines following the UScosequence but shifted to the blue of the sequence to be conservative.We limited our search to sources fainter than 𝑌 = 15 mag becausebrighter sources tend to be saturated in our deep survey. Moreover, ourgoal is to identify the coolest L and T-type members and this regionhas been well surveyed at brighter magnitudes by various teams(e.g. Lodieu et al. 2007a; Dawson et al. 2011; Lodieu et al. 2011a;Luhman & Mamajek 2012; Dawson et al. 2013; Best et al. 2017). Thisselection returned 157 candidates and is abbreviated below as “Newcand 𝑌 𝐽 ” (red dots in all figures). Among these 157, 21 and 16 havematching radii larger than 1.0 and 1.5 arcsec, respectively, yielding alevel of contamination of about 10% due to our conservative pairing.We now proceed to further constrain their membership based onother colour-magnitude diagrams.
We investigated the position of the
𝑌 𝐽 candidates in the ( 𝑍 − 𝐽 , 𝑍 )diagram to further assess their membership (left-hand side panel inFig. 3). We found 153 sources (out of 157) with 𝑍 -band photometry MNRAS , 1–14 (2015) xploring the IPMO population in USco Figure 2.
Left: ( 𝑌 − 𝐽 , 𝑌 ) colour-magnitude diagram for all 𝑌 , 𝐽 sources in the two VISTA surveys with their error bars.
Right:
Same as left but with the additionof known USco members (blue open symbols) from Luhman & Mamajek (2012) and Lodieu et al. (2018a). The thick green line depicts our
𝑌 𝐽 photometricselection criterion. Red symbols highlight the new
𝑌 𝐽 candidates after applying our photometric selection (Section 4.3), whereas the orange symbols and blacktriangles show our best candidates that deserve spectroscopic follow-up. Their ID numbers from the first column of the tables in Appendix A are also indicated.The orange dashed lines represent the BT-Settl models for ages of 10 Myr dashed) and 1 Gyr (dot-dashed). The purple line depicts the sequence of field L and Tdwarfs from Dupuy & Liu (2012). Filled blue triangles and squares show known L and T dwarf members of young moving groups and the Pleiades, respectively.
Figure 3.
Left: ( 𝑍 − 𝐽 , 𝑍 ) colour-magnitude diagram for 𝑌 𝐽 candidates with 𝑍 -band detections (grey dots). Blue open squares and triangles are known UScomembers from Luhman & Mamajek (2012) and Lodieu et al. (2018a). The dashed brown line depicts the 10 Myr-old isochrone from Baraffe et al. (2015). Reddots highlight USco member candidates after applying the selection in the ( 𝑌 − 𝐽 , 𝑌 ) diagram. Orange and black symbols highlight our best L/T transitioncandidates for future spectroscopic follow-up. Right: ( 𝑌 − 𝐽 , 𝑍 − 𝐽 colour-colour diagram with the same legend of symbols.) MNRAS , 1–14 (2015) N. Lodieu et al.
Figure 4. ( 𝑍 − 𝐾 , 𝑍 ) and ( 𝐽 − 𝐾 , 𝐽 ) colour-magnitude diagrams for 𝑌 𝐽 candidates with 𝑍 -band photometry and 𝐾 -band counterparts from the UKIDSSGCS DR10. Red squares and blue triangles are known USco members from Luhman & Mamajek (2012) and Lodieu et al. (2018a). Orange symbols and blacktriangles highlight our final candidates after applying filtering in the 𝑌 𝐽 , 𝑍 𝐽 , and
𝑍 𝐾 colours. The purple line with dots represents the sequence of field M, L,and T dwarfs (Dupuy & Liu 2012). The filled blue triangles show three L7–L8 members of TWA (Gagné et al. 2017a) and SIMP J013656.5+093347 a memberof the 200 Myr-old Carina-Near moving group (Gagné et al. 2017b). The filled blue squares mark the position of the Pleiades member candidates Calar 20, 21,and 22 (Zapatero Osorio et al. 2014c). from the first epoch of our original VISTA
𝑍𝑌 𝐽 survey, the remainingfour are 𝑍 dropouts.Fig. 3 depicts the ( 𝑍 − 𝐽 , 𝑍 ) diagram with the same symbols asthe ( 𝑌 − 𝐽 , 𝑌 ) diagram. We observe that the sequence of members iswell defined and becomes redder down to 𝑍 ∼
20 mag, as confirmedby the compilation of members of Luhman & Mamajek (2012) andhighlighted in our previous studies (e.g. Lodieu et al. 2006, 2007a;Lodieu 2013). Beyond, the sequence seems to flatten with a relativelyconstant 𝑍 − 𝐽 colour and an increased dispersion that might be dueto a combination of physical spread and large error bars at those faintmagnitudes or intrinsic physical properties of such objects (dust,cloud decks).We show the ( 𝑌 − 𝐽 , 𝑍 − 𝐽 ) colour-colour diagram in the right panelof Fig. 3. Based on known members, we can see the USco sequence.We designed two straight lines to discard photometric non-membersbased on these two colours (green dashed lines in the right panel ofFig. 3): the first one goes from ( 𝑌 − 𝐽 , 𝑍 − 𝐽 ) = (0.5,1.1) to (1.5,2.5)mag while the second one excludes all sources with 𝑍 − 𝐽 coloursbluer than 2.5 mag. The former is driven by the position of brightUSco members from Luhman & Mamajek (2012) while the limit onthe 𝑍 − 𝐽 colour corresponds to the reddest field M dwarfs in thesolar neighbourhood (Schmidt et al. 2010). These selection criterialed to 57 bona-fide members highlighted with orange squares in allfigures in this manuscript (Table A1).In Lodieu et al. (2018a) we presented follow-up optical imaging inthe Sloan 𝑖 filter on the OSIRIS instrument (Optical System for Imag-ing and low Resolution Integrated Spectroscopy; Cepa et al. 2000)installed on the Gran Telescopio de Canarias (GTC) in La Palma(Canary Islands). The observations were taken as part of programmeGTC4-14A during the month of August 2014 over several days undervariable conditions resulting in inhomogeneous depths (Fig. 1). We refer the reader to that paper for the logs of the observations andthe description of the data reduction and analysis. We found 10 𝑌 𝐽 candidates covered by the GTC observations, nine previously pub-lished in Lodieu et al. (2018b) and one new. This yet-unpublishedSDSS 𝑖 photometry of VISTA J16125313 − 𝑖 − 𝐽 colour of 2.36 ± 𝑖 = 17.337 ± 𝐽 = 14.977 ± −
153 = 4) 𝑍 dropouts listed in Table A3 in Ap-pendix A, none have additional photometric information from large-scale surveys such as UKIDSS GCS DR9, AllWISE or PanStarrs. Wehave checked the images of these databases and confirm that thosepositions are void of sources down to their detection limits. None ofthese four candidates appear in the unWISE catalogue of Schlaflyet al. (2019). Their 𝑌 and 𝐽 magnitudes range from 22.18–22.67 magand 19.57–20.37 mag, respectively. Except for the source covered bythe Subaru survey, we cannot further constrain the membership ofthese sources so they remain possible candidates. Nonetheless, wehave checked the deep 𝑌 images as well the first VISTA 𝑍, 𝐽 epochand can say the following about each object: • − 𝐽 = 20.25) lies in the halo of a brightstar and might not be a real detection. We cast doubt on the member-ship of this source to USco. • − 𝐽 = 20.37) looks double in the 𝐽 im-ages while the 𝑌 image shows a single source. The object remainsundetected in 𝑍 and in the UKIDSS images. • − 𝐽 = 20.09) lies in the halo of a brightstar but the detections in 𝑌 and 𝐽 are clear. The object is undetectedin 𝑍 and in the images of the UKIDSS GCS. This object remains asa good candidate. • − 𝐽 = 19.57) looks double in the 𝐽 im- MNRAS000
153 = 4) 𝑍 dropouts listed in Table A3 in Ap-pendix A, none have additional photometric information from large-scale surveys such as UKIDSS GCS DR9, AllWISE or PanStarrs. Wehave checked the images of these databases and confirm that thosepositions are void of sources down to their detection limits. None ofthese four candidates appear in the unWISE catalogue of Schlaflyet al. (2019). Their 𝑌 and 𝐽 magnitudes range from 22.18–22.67 magand 19.57–20.37 mag, respectively. Except for the source covered bythe Subaru survey, we cannot further constrain the membership ofthese sources so they remain possible candidates. Nonetheless, wehave checked the deep 𝑌 images as well the first VISTA 𝑍, 𝐽 epochand can say the following about each object: • − 𝐽 = 20.25) lies in the halo of a brightstar and might not be a real detection. We cast doubt on the member-ship of this source to USco. • − 𝐽 = 20.37) looks double in the 𝐽 im-ages while the 𝑌 image shows a single source. The object remainsundetected in 𝑍 and in the UKIDSS images. • − 𝐽 = 20.09) lies in the halo of a brightstar but the detections in 𝑌 and 𝐽 are clear. The object is undetectedin 𝑍 and in the images of the UKIDSS GCS. This object remains asa good candidate. • − 𝐽 = 19.57) looks double in the 𝐽 im- MNRAS000 , 1–14 (2015) xploring the IPMO population in USco Figure 5.
Colour-magnitude diagrams displaying all
𝑌 𝐽 candidates with 𝑍 -band photometry and UKIDSS GCS DR10 and AllWISE/unWISE counterparts.Red squares and blue triangles are known USco members from Luhman & Mamajek (2012) and Lodieu et al. (2018a). Orange symbols highlight our finalcandidates after applying filtering in the 𝑌 𝐽 , 𝑍 𝐽 , and
𝑍 𝐾 colours. Orange symbols and black triangles highlight our best candidates with unWISE photometry.The purple line with dots represents the sequence of field M, L, and T dwarfs (Dupuy & Liu 2012). The blue filled triangles show three L7–L8 members ofTWA (Gagné et al. 2017a) and SIMP J013656.5+093347 a member of the 200 Myr-old Carina-Near moving group (Gagné et al. 2017b) while the blue filledsquares mark the position of the Pleiades member candidates Calar 20, 21, and 22 (Zapatero Osorio et al. 2014c). ages while the 𝑌 image shows a single source. This is a similar caseas the candidate in the second bullet above (16:04:46.7 − 𝑍 image with 𝑍 = 22.959 ± 𝑌 − 𝐽 , 𝑍 − 𝐽 ) and ( 𝑍 − 𝐽 , 𝑍 ) diagrams adds credibilityto its membership to the USco association.Up to now, we have assumed that the USco sequence is clearlyvisible in the ( 𝑌 − 𝐽 , 𝑌 ) diagram and extend previous compilation of astrometric and spectroscopic members. To take into account theblue 𝑌 − 𝐽 colours of field (Section 4.1) and young (Section 4.2)L/T transition brown dwarfs and the extra depth provided by theSubaru HSC survey, we also tried applying a very conservative pho-tometric selection, keeping all sources satisfying 𝑌 − 𝐽 ≥ 𝑌 ≥ 𝑍 -band epoch. Thisselection returned 4248 sources, of which 899 have an entry in theHSC catalogue but only 404 have good quality photometry or are MNRAS , 1–14 (2015)
N. Lodieu et al. unsaturated. Placing these sources in the ( 𝑌 − 𝐽 , 𝑍 − 𝐽 ) diagram, weobserved that five remain as potential candidates because of theirpositions in the diagrams involving 𝑍𝑌 𝐽 photometry (Table A2 inAppendix A). However, only two of these five (the two brightest in 𝑍 ) appear as most likely members because they extend the currentsequence of members in the ( 𝑍 − 𝐽 , 𝑍 ) and ( 𝑌 − 𝐽 , 𝑍 − 𝐽 ) diagrams(Fig. 3). We emphasise that the HSC survey covers only 1.5 squaredegrees and that we recover only ∼
20% (899 out of 4248) of thesources without VISTA 𝑍 photometry. Hence, the number of po-tential L/T transition members in USco might be five times larger,fact that we should consider when discussing the luminosity function(Section 6.2). The
𝑌 𝐽 candidates with 𝑍 -band photometry not rejected in the ( 𝑌 − 𝐽 , 𝑍 − 𝐽 ) diagram, which turns out to be one of the best diagramsto discriminate members from contaminants, remain as potentialmembers in diagrams involving near- and mid-infrared photometryfrom the UKIDSS GCS DR9, AllWISE, and unWISE catalogues,respectively (Fig. 4 and Fig. 5). However, we point out that eightsources with the following ID might be non members based on theirpositions in several of these diagrams with infrared photometry: 𝑌 − 𝐽 and 𝑍 − 𝐽 colours that we observein this part of the sequence (Fig. 4).As mentioned in the previous Section 4.4, only one of the four 𝑌 𝐽 candidates without VISTA 𝑍 detections has additional photometryfrom our Subaru deep survey and remains as bona-fide candidate.The other three sources do not appear in the infrared CMDs (Fig.4–5).Finally, as mentioned earlier, the depth of the 𝐾 and 𝑤 , 𝑤 𝑍 -band photometry from the deepHSC survey, and allow for a fair comparison with the locations of theyoung T-type brown dwarfs in colour-magnitude diagrams. Nonethe-less, we find two of the 10 HSC 𝑍 detections (but VISTA 𝑍 dropouts)in AllWISE, adding credit to their membership because they followthe empirical USCo sequence (Fig. 5). We calculated the proper motion in right ascension and declination ofthe
𝑌 𝐽 candidates comparing the positions between the 𝐽 -band firstepoch and the 𝑌 -band second epoch assuming a mean baseline of 4.15years. We ignore the 𝑍 and 𝑌 first epochs because they are shallowerand do not include a detection for all the 𝑌 𝐽 candidates. We calculatedthe mean dispersion of the full sample of several hundred thousands 𝑌 and 𝐽 detections in both directions to estimate the error bars onthe proper motions of the faintest candidates in our sample. We findmedian absolute deviation values below 10.2 and 9.5 mas/yr forsources brighter than 𝑌 = 21 mag in right ascension and declination,respectively. The deviations increase to values of about 15 mas/yr ,30 mas/yr, and 135 mas/yr in both directions in the 𝑌 = 21–22, 22–23, 23–24 mag intervals, respectively, and represent upper limits. Wenote that the mean values of the proper motions are − Figure 6.
Vector point diagram with the proper motions in the right ascensionand declination (in mas/yr) of the
𝑌 𝐽 candidates (red dots). Typical error barsare drawn in the top left corner. Blue squares and triangles are known UScomembers from the compilation of Luhman & Mamajek (2012) and Lodieuet al. (2018a). Orange symbols highlight our final candidates after applyingfiltering in the
𝑌 𝐽 , 𝑍 𝐽 , and
𝑍 𝐾 colours. motion of USco members from the 4.15 year baseline between thetwo VISTA epochs, yielding ( 𝜇 𝛼 cos 𝛿 , 𝜇 𝛿 ) = ( − − 𝑌 survey. These mean values agree well with the mean values ofthe Hipparcos (de Bruijne et al. 1997; de Zeeuw et al. 1999) and 𝐺𝑎𝑖𝑎 (Luhman & Esplin 2020a) astrometric missions but with largeruncertainties as expected for ground-based proper motions.We list the proper motions for our candidate members in the lasttwo columns of Table A1 in Appendix A and show the vector pointdiagram zoomed on a central part in Fig. 6. The distribution of the bestcandidates, i.e. those remaining after applying filters based on 𝑍 − 𝐽 and 𝑍 − 𝐾 colours, appears to shift towards the expected mean clustermotion in the vector point diagram when compared with candidatesfrom 𝑌 𝐽 colours alone. We observe that most of our photometriccandidates lie within a 3 𝜎 circle centered on the mean proper motionof previous members (yellow circle in Fig. 6). Among the sourcesbrighter than 𝑌 = 19 mag, we would reject 5 and 1 source(s) applyinga 3 𝜎 and 5 𝜎 selection, respectively. The astrometric selection wouldthus suggest a level of contamination of approximately 3.4–17.2%among the 29 sources brighter than 𝑌 = 19 mag. Hence, we can cleardiscard VISTA J16105178 − 𝑌 = 19–20 mag range where the medianabsolute deviations in proper motions remain below 10 mas/yr, wewould reject four sources outside 3 𝜎 , one of them being outside the5 𝜎 limit, VISTA J16153235 − 𝑌 survey with magnitudes fainter than 21.7 mag. Themedian absolute deviations increase drastically beyond 𝑌 = 22 mag,as reported above. MNRAS000
𝑍 𝐾 colours. motion of USco members from the 4.15 year baseline between thetwo VISTA epochs, yielding ( 𝜇 𝛼 cos 𝛿 , 𝜇 𝛿 ) = ( − − 𝑌 survey. These mean values agree well with the mean values ofthe Hipparcos (de Bruijne et al. 1997; de Zeeuw et al. 1999) and 𝐺𝑎𝑖𝑎 (Luhman & Esplin 2020a) astrometric missions but with largeruncertainties as expected for ground-based proper motions.We list the proper motions for our candidate members in the lasttwo columns of Table A1 in Appendix A and show the vector pointdiagram zoomed on a central part in Fig. 6. The distribution of the bestcandidates, i.e. those remaining after applying filters based on 𝑍 − 𝐽 and 𝑍 − 𝐾 colours, appears to shift towards the expected mean clustermotion in the vector point diagram when compared with candidatesfrom 𝑌 𝐽 colours alone. We observe that most of our photometriccandidates lie within a 3 𝜎 circle centered on the mean proper motionof previous members (yellow circle in Fig. 6). Among the sourcesbrighter than 𝑌 = 19 mag, we would reject 5 and 1 source(s) applyinga 3 𝜎 and 5 𝜎 selection, respectively. The astrometric selection wouldthus suggest a level of contamination of approximately 3.4–17.2%among the 29 sources brighter than 𝑌 = 19 mag. Hence, we can cleardiscard VISTA J16105178 − 𝑌 = 19–20 mag range where the medianabsolute deviations in proper motions remain below 10 mas/yr, wewould reject four sources outside 3 𝜎 , one of them being outside the5 𝜎 limit, VISTA J16153235 − 𝑌 survey with magnitudes fainter than 21.7 mag. Themedian absolute deviations increase drastically beyond 𝑌 = 22 mag,as reported above. MNRAS000 , 1–14 (2015) xploring the IPMO population in USco We conducted spectroscopy of two candidates with the X-shooter(D’Odorico et al. 2006; Vernet et al. 2011) instrument on the ESOVery Large Telescope (VLT) Unit 2 in visitor mode on the night of 7May 2018 (programme 0101.C-0565; PI Lodieu). The combinationof the faintest of the targets, time allocation, and weather condi-tions allowed us to collect spectra for only two of our photometriccandidates with sufficient signal-to-noise for spectral classification(Table 2). X-shooter is a multi-wavelength cross–dispersed echellespectrograph equipped with three independent arms observing si-multaneously in the ultraviolet (UVB; 0.3–0.56 𝜇 m), visible (VIS;0.56–1.02 𝜇 m), and near–infrared (NIR; 1.02–2.48 𝜇 m) whose lightis split by two dichroics. The UVB, VIS, and NIR arms are equippedwith a 4096 × × × 𝑌 magnitudes, date of the observations, numberof exposures, and exposure times set in the NIR arm for both targets. We downloaded the reduced 2D spectra of the two targets from theESO science archive (Table 2). The data reduction is made automat-ically with the esoreflex pipeline and includes 2D and 1D spectraextracted optimally without telluric correction. We did not detectany signal in the UVB arm and no obvious emission line like H 𝛼 in the VIS spectra of both targets, except for extremely weak signalsbeyond 900 nm, hence, we concentrate our analysis on the NIR arm.We removed tellurics from the NIR spectra of both targets with the molecfit package distributed by ESO (Kausch et al. 2015; Smetteet al. 2015) . The final spectra are shown in Fig. 7 with the smoothedspectra by a factor of 21 pixels (black) on top of the unsmoothedspectra displayed in grey. We inspected the X-shooter infrared spectra displayed in Fig. 7,which reveal the presence of spectral features typical of young (i.e.low gravity) L dwarfs: a strong VO band at 1.06 micron, the trian-gular shape in the 𝐻 -band, weak alkali lines like the K I potassiumdoublets at 1.169/1.177 and 1.243/1.252 microns, and strong waterbands (Martín et al. 1996; Luhman et al. 1998; Zapatero Osorio Table 2.
Logs of the VLT X-shooter spectroscopic observations. We providethe identified number (Table A1), coordinates (J2000) of the targets, their 𝑌 -band magnitudes, the date of observations, and the number and length ofthe on-source integrations employed for the NIR arm.ID RA (J2000) dec (J2000) 𝑌 Date ExpThh:mm:ss.ss dd:’:” mag yyyy-mm-dd sec15 16:06:09.23 − ± × − ± × Figure 7.
VLT X-shooter spectra of two of our best photometric candidates.Top figure: we plot USco 16060923 − − − − 𝐽 -band panel, both sensitive to gravity. et al. 2000; Lucas & Roche 2000; Gorlova et al. 2003; McGov-ern et al. 2004; Kirkpatrick 2005; Cruz et al. 2009; Allers & Liu2013; Alves de Oliveira et al. 2013; Bonnefoy et al. 2014; Manjava-cas et al. 2014; Mužić et al. 2014; Lodieu et al. 2018a; Chinchillaet al. 2020). We performed the spectral classification of the twosources by direct comparison with known L0–L7 dwarf members ofthe USco association collected with the same VLT/X-shooter set-up(Lodieu et al. 2018a) and Gemini Near-Infrared Spectrograph GNIRS(Elias et al. 2006; Lodieu et al. 2008). We infer infrared spectraltypes of L1.0 ± ± − − I potassium doublets presentin the 𝐽 -band region of the spectra for both objects (left pan-els in Fig. 7). We set upper limits of 0.01 nm on the doubletat (1169.0,1173.3) nm. We derived pseudo-equivalent widths of(0.031 ± ± ± ± MNRAS , 1–14 (2015) N. Lodieu et al.
Old field T dwarfs are brown dwarfs whose infrared spectra areshaped by strong methane and water bands as well as H collision-induced absorption (Leggett et al. 2000; Burgasser et al. 2002;Geballe et al. 2002; Burgasser et al. 2006). Their optical spectraare dominated by pressure-broadened alkali lines (Burgasser et al.2003). The dust present in L dwarfs has settled at the bottom of theiratmospheres, resulting in red optical and optical-to-infrared coloursbut blue near-infrared colours. Their effective temperatures are typi-cally below 1300 K.In view of the numbers of possible candidate members left afterapplying the difference colours cuts, we discuss the chances that thesemight be T-type dwarfs belonging to USco. Based on photometryand colours, field L and T dwarfs display constant 𝑌 − 𝐽 colours withsome significant scatter (Hewett et al. 2006; Lodieu et al. 2007b;Pinfield et al. 2008; Burningham et al. 2010, 2013), opposite tothe sequence of USco member candidates identified in the ( 𝑌 − 𝐽 , 𝑌 )colour-magnitude diagram (Fig. 2). We observe that the 𝑌 − 𝐽 coloursof potential late-L/early-T member candidates is not constant butrather becoming redder with fainter magnitudes. This is expected formembers of a cluster or association that follow a sequence, with thecoolest members exhibiting the latest spectral types.The 𝑍 − 𝐽 colours of field L/T dwarfs remain relatively constant,consistent with most of our USco candidates. Nonetheless, we identi-fied a few sources with distinct 𝑍 − 𝐽 colours compared to other UScocandidates (Fig. 3). On the one hand, USco J1611015 − 𝑍 − 𝐽 but rather blue in 𝑌 − 𝐽 . On the otherhand, USco J16051784 − 𝐽 − 𝐾 colour (purple dashed line inFig. 4) based on the absolute magnitude vs spectral type relationsof Dupuy & Liu (2012) in the Mauna Kea Observatory system aswell as the 𝐽 − 𝑤 𝐽 − 𝑤 𝐾 − 𝑤 𝑤 − 𝑤 𝐾 -band (100% completeness of about 18.2 mag) andthe AllWISE/unWISE catalogues that do not provide any detectionfor our faintest candidates. This suggests that our survey has not yetuncovered T-type planetary-mass objects in USco.Moreover, we note that the 𝐽 -band magnitudes of the faintest can-didates are similar because the 𝑌 magnitudes become fainter and the 𝑌 − 𝐽 redder at the same time. Consequently, their absolute magni-tudes should be comparable, which we would expect if the trend isconsistent with field L6–T4 dwarfs having 𝐽 -band absolute magni-tudes that differ by less than a few tens of magnitudes (Dupuy & Liu2012). Hence, the new candidates might well be late-L dwarfs butcould also have later spectral types (e.g. cases of maybe ) because we lack unambigu-ous evidence in spite of being two magnitudes fainter in 𝑌 andhaving identified candidates one magnitude fainter in 𝐽 than late- L dwarfs confirmed spectroscopically. Nonetheless, we highlightUSco J16051784 − 𝑌 − 𝐽 and 𝑍 − 𝐽 colours that differ from othercandidates and that an increase redness. Moreover, we cannot dis-card USco J1611015 − 𝑍 − 𝐽 colours consistent with the synthetic colours ofold field T dwarfs (Hewett et al. 2006). However, further photometryand spectroscopy are needed to confirm both as young T dwarfs. The aim of this subsection is to discuss the shape and form of theUSco luminosity and mass functions into the planetary-mass regime.In Table 3, we give the final numbers of
𝑌 𝐽 candidates before andafter rejection of potential photometric non members, equivalentto the luminosity function (Fig. 8). We transform the luminosityfunction into a mass spectrum, assuming an age of 10 Myr (Pecautet al. 2012) and the magnitude-mass relation from the BT-Settl model(Baraffe et al. 2015). We also specify the mass range according tothe BT-Settl 10 Myr-old isochrone in Table 3. We assumed an age of10 Myr for USco based on the comparison of these models with themasses derived from eclipsing binaries identified in the associationthanks to the Kepler K2 mission (Borucki et al. 2010; Lissauer et al.2014; Batalha 2014) over the past years (Alonso et al. 2015; Krauset al. 2015; Lodieu et al. 2015; David et al. 2016b, 2019). The BT-Settl models at 10 Myr reproduce relatively well the sequence ofUSco eclipsing binaries in the mass-radius diagram (Lodieu et al.2020). The age of USco might well be as young as 5 Myr fromisochrone fitting of the lowest mass stars (Preibisch & Zinnecker1999; Preibisch et al. 2001; Song et al. 2012; David et al. 2019) withan upper limit of about 10 Myr (Pecaut et al. 2012).In Table 3 we observe that the number of objects per bin of magni-tudes in the six square degrees is relatively constant over the 𝐽 = 14–20 mag range. We considered Poissonian errors on the luminosityfunction taking the square root of the number of objects in each col-umn that we need to sum up to obtain the total number of candidatesper magnitude bin. This is as a very approximate but recommendedpractice to estimate the error bars in histograms . The last two binsare incomplete because our 𝐽 -band survey is 100% complete downto 𝐽 = 20.5 mag. Typically we find 1.0–1.5 planetary-mass memberper square degree in this area of USco, up to two maximum. Weobserve an increase in the planetary-mass regime if all photometric 𝑌 𝐽 candidates are bona-fide members. If only the smallest numbersof candidates is confirmed with further follow-ups, the shape of themass spectrum would rather be flat in the 20–7 M
Jup range. We candiscard a significant decrease in the numbers of members down to 5M
Jup , assuming an age of 10 Myr, unless all photometric candidatesare rejected in the future. If the age of USco is 5 Myr, the lowermass limit of our survey would be just below 4 M
Jup . In spite of alluncertainties associated to models at these low masses and youngages as well as the lack of spectroscopic confirmation, star formationprocesses can form isolated planetary-mass objects down to 5 M
Jup ,consistent with the findings in the solar neighbourhood (Kirkpatricket al. 2019). https://docs.astropy.org/en/stable/api/astropy.stats.poisson_conf_interval.html MNRAS000
Jup ,consistent with the findings in the solar neighbourhood (Kirkpatricket al. 2019). https://docs.astropy.org/en/stable/api/astropy.stats.poisson_conf_interval.html MNRAS000 , 1–14 (2015) xploring the IPMO population in USco Table 3.
Numbers of new
𝑌 𝐽 candidates per bin of 𝐽 magnitudes. We list the total number of objects (= luminosity function) in the area of six square degreescovered by our deep 𝑌 survey. We give the numbers of 𝑌 𝐽 candidates before (
𝑌 𝐽 ) and after rejection (Rej) based on their position in diagrams involvingadditional passbands. We also list the four candidates undetected in the VISTA 𝑍 survey (no 𝑍 ) and those remaining as potential candidates from the deep HSCsurvey (HSC 𝑍 ). We take into account that the Subaru survey covers only 20% of the VISTA survey, resulting in a factor of five to consider when counting thenumber of potential candidates after applying the selection in the 𝑧 -band. The final number (or range) of objects (dN) is the sum of columns 2, 4, and 5 minusthe number of rejected candidates in column 3. We give in parenthesis the range of values considering the Poissonian statistics rounded to the nearest decimal.The mass range is computed using the BT-Settl 10 Myr-old isochrones. The last column gives the number of objects per mass bin rounded to the nearest integer,dividing dN by the difference of the mass range with its associated Poissonian interval in parenthesis. We warn that the first and last two bins are incomplete dueto saturation at bright magnitude and the 100% completeness at 𝐽 = 20.5 mag, respectively. 𝐽 𝑌 𝐽
Rej no 𝑍 HSC 𝑍 dN Mass range dN/dMmag M (cid:12) −
15 10 0 0 0 10 (6.8–13.1) 0.0600–0.0220 263 (247–279)15 −
16 7 1 0 0 6 (3.5–8.5) 0.0220–0.0160 1000 (968–1032)16 −
17 8 3 0 0 5 (2.7–7.2) 0.0160–0.0120 1250 (1214–1285)17 −
18 9 3 0 0 6 (3.5–8.5) 0.0120–0.0095 2400 (2351–2449)18 −
19 13 5–8 0 0 5–8 (2.7–10.3) 0.0095–0.0073 2242–3587 (2194–3647)19 −
20 9 0 0–1 1 × −
21 1 0 1–2 1 × −
22 0 0 0 (2 ± × ≥ (5–15) ≥ (2.9–17.1) 0.0045–0.0037 ≥ Figure 8.
Luminosity function: numbers of new
𝑌 𝐽 candidates in the 𝐽 = 14 −
21 mag interval before rejection of potential photometric non mem-bers (black). The red and magenta histograms indicate the minimum andmaximum numbers of
𝑌 𝐽 candidates after rejection of potential photometricnon members (Table 3). The first and last two bins are incomplete due tosaturation at bright magnitude and the 100% completeness at 𝐽 = 20.5 mag,respectively. We presented a dedicated photometric search in ∼ in the centralregion of the USco association to look for the coolest members witha special focus on late-L and T-type candidates. Our survey is thedeepest in the association over such large area, between 2 and 1magnitude deeper in 𝑌 and 𝐽 than any previous study and we are nowcloser to reach the regime of T dwarfs in USco. The main results ofour study are: • We identified 10 new candidates fainter than the previouslyknown L7 members confirmed spectroscopically. Their magnitudesare fainter than 𝑌 = 21.7 mag ( 𝐽 >
Jup according to evolutionary models at an age of 10 Myr. • We highlight two potential T-type member candidates amongthe 10 faintest sources identified in our deep survey. • We derived proper motion confirmation of the brightest membercandidates while the accuracy of the astrometry is limited for thenewest and faintest candidates. • We presented near-infrared spectroscopy of two photometriccandidates confirmed them as young L dwarfs, adding credence totheir membership. The remaining candidates require optical and/orinfrared spectroscopic follow-up. • We derive photometric estimates of the luminosity function andmass spectrum in the planetary-mass domain showing no obvioussign of dearth of members down to masses of 5 M
Jup . Although thisanalysis is preliminary and requires photometric and spectroscopicfollow-ups, there is now growing observational evidence that the starformation processes can form isolated objects with masses below 5M
Jup in the field and in young regions.
ACKNOWLEDGMENTS
NL was partly funded by the programme AYA2015-69350-C3-2-Pfrom Spanish Ministry of Economy and Competitiveness (MINECO)and acknowledges support from the Agencia Estatal de Investigacióndel Ministerio de Ciencia e Innovación (AEI-MCINN) under grantPID2019-109522GB-C53. NL has benefited from internal fundingfrom an IAC Severo Ochoa outgoing fellowship for a stay at the RoyalObservatory in Edinburgh. We warmly thank the staff at the RoyalObservatory Edinburgh and Cambridge Astronomy Survey Unit fortheir valuable help and support with the data reduction, in particularMike Read, Rob Blake, Eckhard Sutorius, Mike Irwin, Aybuke KupcuYoldas, and Carlos González-Fernańdez.This research has made use of the Simbad and Vizier (Ochsen-bein et al. 2000) databases, operated at the Centre de Données As-tronomiques de Strasbourg (CDS), and of NASA’s Astrophysics DataSystem Bibliographic Services (ADS).Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere under ESO pro-gramme(s) 095.C-0781(A), 089-C.0102(ABC), and 0101.C-0565.This work is based on programmes GTC4-14A (PI Lodieu) made
MNRAS , 1–14 (2015) N. Lodieu et al.
DATA AVAILABILITY
The data underlying this article will be made public to the communitythrough Vizier at the Centre de Données de Strasbourg at http://cdsweb.u-strasbg.fr , and can be accessed with the bibcodeor last name of the first author of the paper.
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APPENDIX A: TABLES WITH LIST OF CANDIDATES
This paper has been typeset from a TEX/L A TEX file prepared by the author.MNRAS000
This paper has been typeset from a TEX/L A TEX file prepared by the author.MNRAS000 , 1–14 (2015) xploring the IPMO population in USco Table A1.
List of USco member candidates that passed our photometric selection criteria ordered by 𝑌 magnitude. We list the coordinates in the second epochdeep 𝑌 survey (in J2000), optical (deep VISTA 𝑍 survey), near-infrared ( 𝑌 , 𝐽 from VISTA;
𝐻 , 𝐾 from UKIDSS GCS DR9), and mid-infrared ( 𝑤 , 𝑤 𝑌 𝐽 𝑍 𝐻 𝐾 𝑤 𝑤 𝜇 𝛼 cos ( 𝛿 ) 𝜇 𝛿 hh:mm:ss.ss ◦ : (cid:48) : (cid:48)(cid:48) mag mag mag mag mag mag mag mas/yr mas/yr53 16:03:50.82 − ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± − − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± − − − ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − ± ± ± ± ± − − ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − ± ± ± ± ± ± ± − − ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± ± ± − − − ± ± ± ± ± − − ± ± ± ± ± − − − ± ± ± ± ± − − ± ± ± − − ± ± ± ± ± ± ± − − ± ± ± ± ± ± − − ± ± ± − − ± ± ± − − − ± ± ± ± − − − ± ± ± − − − ± ± ± ± − − ± ± ± ± ± − − , 1–14 (2015) N. Lodieu et al.
Table A2.
List of USco member
𝑌 𝐽 candidates undetected in the deep VISTA 𝑍 -band survey. They are among the 10 newly identified potential candidatesfainter than the coolest members knonw in USco. None of these candidates has counterparts in UKIDSS GCS DR9, AllWISE, and PanStarrs. Only the lastobject in the table has a 𝑍 magnitude from the deep Subaru/HSC survey ( 𝑍 = 22.959 ± 𝑌 survey (in J2000), 𝑌 𝐽 photometry with their error bars, and the proper motion in mas/yr measured between the first ( 𝐽 ) and second ( 𝑌 ) epochVISTA surveys. ID R.A. Dec 𝑌 𝐽 𝜇 𝛼 cos 𝛿 𝜇 𝛿 hh:mm:ss.ss ◦ : (cid:48) : (cid:48)(cid:48) mag mag mas/yr mas/yrcandYJ_VISTA_101 16:09:13.79 − ± ± − − ± ± − − ± ± − − ± ± − Table A3.
List of USco member
𝑌 𝐽 candidates with detections in the Subaru/HSC 𝑍 -band survey. We give their ID number, coordinates in the second epochdeep 𝑌 survey (in J2000), Subaru 𝑍 photometry, 𝑌 𝐽 photometry with their error bars. The two brightest in 𝑍 appear as the best candidates because they extendthe USco sequence shown in colour-colour diagrams involving the 𝑍𝑌 𝐽 filters.ID R.A. Dec
𝑍 𝑌 𝐽 hh:mm:ss.ss ◦ : (cid:48) : (cid:48)(cid:48) mag mag magcandYJ_HSC_1005 16:12:03.71 − ± ± ± − ± ± ± − ± ± ± − ± ± ± − ± ± ±000