Natalia Oliveros-Gomez ,1, 2 Elena Manjavacas ,3, 2 Daniella C. Bardalez Gagliuffi ,4, 5Theodora Karalidi ,6 Johanna M. Vos ,7, 8 and Jacqueline K. Faherty 51- Departamento de Astronom´ıa, Universidad de Guanajuato. Callej´on de Jalisco, S/N, 36023, Guanajuato, GTO, M´exico2- William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA3- AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD,21218 USA4- Department of Physics & Astronomy, Amherst College, 25 East Drive, Amherst, MA 01003, USA5- Department of Astrophysics, American Museum of Natural History 200 Central Park West, New York, NY 10024, USA6- Department of Physics, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, USA7- School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland8Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USAABSTRACTMost brown dwarfs show some level of photometric or spectral variability. However, finding the most variable dwarfs more suited for a thorough variability monitoring campaign remained a challenge until a few years ago with the design of spectral indices to find the most likely L and T dwarfs using their near-infrared single-epoch spectrum. In this work, we designed and tested near-infrared spectral indices to pre-select the most likely variable L4-L8 dwarfs, complementing the indices presented by Ashraf et al. (2022) and Oliveros-Gomez et al. (2022). We used time-resolved near-infrared Hubble Space Telescope Wide Field Camera 3 spectra of a L6.0 dwarf, LP 261–75b, to design our novel spectral indices. We tested these spectral indices on 75 L4.0-L8.0 near-infrared SpeX/IRTF spectra, providing 27 new variable candidates. Our indices have a recovery rate of ∼80%, and a false negative rate of ∼25%. All the known non-variable brown dwarfs were found to be non-variable by our indices. We estimated the variability fraction of our sample to be 51+4 −38%, which agrees with the variability fractions provided by Buenzli et al. (2014), Radigan et al. (2014) and Metchev et al. (2015) for L4–L8 dwarfs. These spectral indices may support in the future the selection of the most likely variable directly-imaged exoplanets for studies with the James Webb Space Telescope and as well as the 30-m telescopes.
J2224-0158 is an L4.5V dwarf (Kirkpatrick et al. 2000). It is located at a distance of 11.6075 0.0549 pc (Gaia Collaboration 2020). Burningham et al. (2021) found an effective temperature of Teff = 1912 K, and a surface gravity of log(g) = 5.47 dex using atmospheric retrievals. Metchev et al. (2015) did not find variability on this object. Gelino et al. (2002) did not measure any variability in the I-band for J2224-0158. In addition, Burningham et al. (2021) showed us that it has enstatite and quartz clouds, but that the body may have an unfavorable geometry to detect rotational modulation signals, or that the dark regions are arranged latitudinally in bands, and this may be why variability in near-infrared wavelengths is not observed. J2224-0158 was found in the variable areas in 6 out of 15 of our indexindex plots, indicating that it is a non-variable candidate.
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Buenzli et al. (2014) measured a ramp in the 1.12-1.2 m wavelength range, with a small degree of variability, and they presented J1219+3128 as a tentative variable. J1219+3128 was found in the variable areas in 6 out 5.2.10. False Negatives In this section, we describe the objects that we determine as false negatives because our spectral index method determines them as non-variable objects, but in 11 12 Oliveros-Gomez et al. the literature, they have been measured to have variability in different spectral bands. 2MASSW J0030300-145033: J0030-1450 is an L7 dwarf (Kirkpatrick et al. 2000). It is located at a distance of 26.7 3.3 pc (Faherty et al. 2009). Gagne et al. (2015) found an effective temperature of Teff = 1500 K, and a surface gravity of log(g) = 5.0 dex by fitting the SED of the object using the BT-Settl atmospheric models.
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Enoch et al. (2003) detected periodic variability in Ks-band, with an amplitude of 0.19 mag and period of 1.5 hr. However Clarke et al. (2008) found no evidence of variability, with amplitude smaller than 40 mmag in J-band. Similarly, Radigan et al. (2014) observed a flat light curve for this target in the J-band throughout 3.2 hr. Vos et al. (2022) found variability of A[3.5m] = 1.52 0.06% using Spitzer and obtained a period of 4.22 0.02 hr. Therefore, despite discrepancies among the literature results, they are not mutually exclusive. As the observations are not simultaneous, this may be an example where the extent of variability varies over time. J0030-1450 was found in the variable areas in 5 out of 15 of our indexindex plots, indicating that it is a non-variable candidate. This may be a false negative of our sample.
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2MASS J10101480-0406499: J1010-0406 is field L6 dwarf (Cruz et al. 2003; Reid et al. 2008). It is located at a distance of 18 2 pc. Gagne et al. (2015) found an effective temperature of Teff = 1600 K, and a surface gravity of log(g) = 5.0 dex by fitting the SED of the object using the BT-Settl atmospheric models. Wilson et al. (2014) found a variability amplitude of 5.1 1% in an observation window of only 3 hr, using the SofI instrument (Jband). They did not report a period, because the window observation is too short for a proper determination. J1010-0406 was found in the variable areas in 6 out of 15 of our indexindex plots, indicating that it is a non-variable candidate. This is a false negative object in our sample. 2MASS J22443167+2043433: J2244+2043 is an L6.5gamma (Faherty et al. 2016), young and low-gravity dwarf (Gizis et al. 2015). It is located at a distance of 19 2 pc (Faherty et al. 2009). Faherty et al. (2016) found an effective temperature of Teff = 1184 K.
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