Rotation periods and photometric variability of rapidly rotating ultra-cool dwarfs
P. A. Miles-Páez E. Pallé M. R. Zapatero Osorio


firstpage–lastpage
Analyticity Constraints for Hadron Amplitudes: 

Going High to Heal Low Energy Issues
    G. Fox
    December 30, 2023
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 We used the optical and near-infrared imagers located on the Liverpool, the IAC80, and the William Herschel telescopes to monitor 18 M7-L9.5 dwarfs with the objective of measuring their rotation periods. We achieved accuracies typically in the range ±1.5–28 mmag by means of differential photometry, which allowed us to detect photometric variability at the 2σ level in the 50% of the sample. We also detected periodic modulation with periods in the interval 1.5–4.4 h in 9 out of 18 dwarfs that we attribute to rotation. Our variability detections were combined with data from the literature; we found that 65 ± 18 % of M7–L3.5 dwarfs with v sin i≥30 km s^-1 exhibit photometric variability with typical amplitudes ≤20 mmag in the I-band. For those targets and field ultra-cool dwarfs with measurements of v sin i and rotation period we derived the expected inclination angle of their rotation axis, and found that those with v sin i≥30 km s^-1 are more likely to have inclinations ≳40 deg. In addition, we used these rotation periods and others from the literature to study the likely relationship between rotation and linear polarization in dusty ultra-cool dwarfs. We found a correlation between short rotation periods and large values of linear polarization at optical and near-infrared wavelengths.





polarization – brown dwarfs – stars: atmospheres – stars: late-type – stars: low-mass








§ INTRODUCTION


The presence of magnetic spots and/or dust clouds in the atmosphere of very low-mass stars and brown dwarfs (“ultra-cool dwarfs”, SpT≥M7) has been proposed to account for the spectro-photometric variability observed by different groups <cit.>. These structures can modulate the shape of the light curve as the dwarf rotates. In some cases, the shape of the observed light curve does not vary significantly with each rotation <cit.>, while in other cases, changes in the shape and amplitude are observed after a few rotations <cit.>. Infrared observations from space <cit.> and the ground <cit.> have shown that the amplitudes and occurrence rates of variability do not decrease with spectral type. The observed amplitudes of photometric variability  are typically ≤ 1.5 % at optical and infrared wavelengths—even though amplitudes ≥ 2 % have been observed in some L/T dwarfs—with periodicities in the range ≈1.4–20 h, which have been attributed to rotation.  
 
Magnetic activity was one of the first mechanisms proposed to account for the observed photometric variability. Hα observations—a sign of magnetic activity—have revealed that nearly 100% of M7–L3.5 dwarfs are magnetically active <cit.>, so the presence of magnetic spots could lead to photometric variability as in stars <cit.>. In addition, optical and radio aurorae have been proposed as another possible origin for some of the variability observed at optical and near-infrared wavelengths <cit.>. However, strong magnetic fields are not common in ultra-cool dwarfs, especially in those cooler than spectral type L3.5 <cit.>, so magnetic activity alone cannot explain the photometric variability observed from late-M to T dwarfs. 

The low temperatures in the upper atmospheric layers of ultra-cool dwarfs favor the natural formation of condensates <cit.>, which can also induce photometric variability. State-of-the-art radiative transfer models have shown that the observed photometric variability in several ultra-cool dwarfs cannot be explained only by magnetic spots, unless they are combined with dust clouds <cit.>. Recently, <cit.> investigated a sample of 94 ultra-cool dwarfs, all of them with optical spectra containing the Hα region and a photometric monitoring with a duration of at least a few hours. These authors found that the detection rate of magnetic activity and photometric variability follow different dependencies as a function of spectral type, and concluded that i) both phenomena are uncorrelated, and ii) that dust clouds are the most likely origin for the observed photometric variability, even for the warmest ultra-cool dwarfs.
 
The presence of photospheric dust can linearly polarize the output flux of the ultra-cool dwarf by means of scattering processes. This was theoretically studied by <cit.>, <cit.>, <cit.>, and <cit.>. These works predicted values of linear polarization ≲2% at optical and near-infrared wavelengths in good agreement with observations in the filters R, I, Z, J, and/or H <cit.>. Non-zero net linear polarization is detectable provided an asymmetry on the surface of the dwarf avoids the cancellation of the polarimetric signal from different areas of the visible disk, such as an oblate shape and/or an heterogeneous dust distribution. Both of these asymmetries can be present in ultra-cool dwarfs at the same time; however, their effects on the net polarization are different as the dwarf rotates. For an inhomogeneous atmosphere, the polarimetric signal is expected to modulate with rotation <cit.>, while for an asymmetry produced by a non-spherical shape, the most oblate ultra-cool dwarfs show the largest values of linear polarization, but no changes are expected in the signal of linear polarization as the dwarf rotates. <cit.> studied the J-band linear polarimetric properties of a sample of rapidly rotating ultra-cool dwarfs (v sin i≥30 km s^-1), finding that dwarfs with v sin i≥60 km s^-1 show statistically larger values of linear polarization than those with smaller v sin i.

Here, we present the results of our optical (I and SDSS-i) and near-infrared (J) photometric monitoring of a sample of 18 M7-L9.5  rapidly-rotating ultra-cool dwarfs by using the Liverpool, the IAC80, and the William Herschel telescopes. Our sample is presented in Section <ref>, and our data acquisition and reduction are described in Section <ref>. In Section <ref>, we describe how the differential intensity light curves of the targets were derived, and how we searched for periodic photometric variability. In Section <ref>, the rotation periods (T_ rot) found for our sample are compared with others from the literature, and we use them to derive the likely inclination angle of the rotation axis of our targets. Finally in Section <ref>, we investigate the likely relationship between short rotation periods and large values of linear polarization for some of our targets that have linear polarimetric data acquired with similar filters. Our conclusions are summarized in Section <ref>.




