J/AJ/165/181 Polarization monitoring of 2MJ21392676+0220226 (Manjavacas+, 2023)
Time-resolved Optical Polarization Monitoring of the Most Variable Brown Dwarf.
Manjavacas E., Miles-Paez P.A., Karalidi T., Vos J.M., Galloway M.L.,
Girard J.H.
<Astron. J., 165, 181 (2023)>
=2023AJ....165..181M 2023AJ....165..181M
ADC_Keywords: Stars, variable; Stars, brown dwarf; Photometry, infrared
Keywords: Brown dwarfs ; T dwarfs ; Starlight polarization ; Atmospheric
variability
Abstract:
Recent atmospheric models for brown dwarfs suggest that the existence
of clouds in substellar objects is not needed to reproduce their
spectra, nor their rotationally induced photometric variability,
believed to be due to the heterogeneous cloud coverage of brown dwarf
atmospheres. Cloud-free atmospheric models also predict that their
flux should not be polarized, as polarization is produced by the light
scattering of particles in the inhomogeneous cloud layers of brown
dwarf atmospheres. To shed light on this dichotomy, we monitored the
linear polarization and photometric variability of the most variable
brown dwarf, 2MASSJ21392676+0220226. We used FORS2 at the UT1
telescope to monitor the object in the z band for six hours, split on
two consecutive nights, covering one-third of its rotation period. We
obtained the Stokes parameters, and we derived its time-resolved
linear polarization, for which we did not find significant linear
polarization (P=0.14%±0.07%). We modeled the linear polarimetric
signal expected assuming a map with one or two spot-like features and
two bands using a polarization-enabled radiative transfer code. We
obtained values compatible with the time-resolved polarimetry obtained
for 2MASSJ21392676+0220226. The lack of significant polarization might
be due to photometric variability produced mostly by banded structures
or small-scale vortices, which cancel out the polarimetric signal from
different regions of the dwarf's disk. Alternatively, the lack of
clouds in 2MASSJ21392676+0220226 would also explain the lack of
polarization. Further linear polarimetric monitoring of
2MASSJ21392676+0220226, during at least one full rotational period,
would help to confirm or discard the existence of clouds in its
atmosphere.
Description:
We conducted linear polarimetric imaging photometry of 2M2139+0220 by
using the zSPECIAL+43 filter and the FOcal Reducer and low-dispersion
Spectrograph 2 (FORS2), which is mounted on the Antu unit (UT1) of the
Very Large Telescope (VLT) of the European Southern Observatory (ESO)
in Cerro Paranal, Chile. FORS2 is by default equipped with a detector
system that is optimized for the red with a very low level of fringes
thanks to a mosaic of two 2048x4096MIT CCDs (with 15µm pixels). The
plate scale is 0.252"pixel for the standard readout mode (2x2
binning). The central wavelength and the passband of the zSPECIAL+43
filter are 916 and 18nm, respectively.
Objects:
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RA (2000) DE Designation(s)
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21 39 26.76 +02 20 22.6 2M2139+0220 = 2MASS J21392676+0220226
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
fig2.dat 36 124 Q, U, P, and corrected polarization
fig11.dat 38 124 Corrected light curves of 2M2139+0220 in the
ordinary beams for night 1 and 2
fig12.dat 38 124 Corrected light curves of 2M2139+0220 in the
extraordinary beams for night 1 and 2
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See also:
J/ApJ/637/1067 : NearIR spectral classification of Tdwarfs (Burgasser+, 2006)
J/ApJS/201/19 : Hawaii Infrared Parallax Program. I. (Dupuy+, 2012)
J/A+A/562/A127 : NIR integral field spectra of 15 M-Ldwarfs (Bonnefoy+, 2014)
J/A+A/564/A55 : 1.1-2.4um spectra of 7 young M & Ldwarfs (Manjavacas+, 2014)
J/ApJ/814/65 : Jupiter HST light curves (Karalidi+, 2015)
J/AJ/160/38 : Spitzer variability detections 79 brown dwarfs (Vos+, 2020)
J/MNRAS/513/5701 : Silicate clouds in ultracool dwarfs (Suarez+, 2022)
Byte-by-byte Description of file: fig2.dat
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Bytes Format Units Label Explanations
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1- 2 A2 --- Type Observation type (1)
4- 17 F14.8 d MJD [59508/59510] Modified Julian Date of
observation; MJD-2400000.5
19- 27 E9.2 % Percent [-4.55/37.6] Percent in Type
29- 36 E8.2 % e_Percent [0.36/0.79] Uncertainty in Percent
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Note (1): Types as follows:
P = linear polarization (31 occurrences)
Pc = Corrected linear polarization (31 occurrences)
Q = Stokes Q parameter (31 occurrences)
U = Stokes U parameter (31 occurrences)
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Byte-by-byte Description of file: fig1[12].dat
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Bytes Format Units Label Explanations
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1- 4 F4.1 deg Beam Beam angle
6- 19 F14.8 d MJD Modified Julian Date of the observation;
MJD-2400000.5
21- 29 E9.2 ct Ct Observed counts
31- 38 E8.2 ct e_Ct Uncertainty in Ct
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History:
From electronic version of the journal
(End) Prepared by [AAS], Coralie Fix [CDS], 15-Sep-2023