The Y-Type Brown Dwarfs: Estimates of Mass and Age from New Astrometry, Homogenized Photometry and Near-Infrared Spectroscopy (1704.03573v1)

Abstract: The survey of the mid-infrared sky by the Wide-field Infrared Survey Explorer (WISE) led to the discovery of extremely cold low-mass brown dwarfs, classified as Y dwarfs, which extend the T class to lower temperatures. Twenty-four Y dwarfs are known at the time of writing. Here we present improved parallaxes for four of these, determined using Spitzer images. We give new photometry for four late-type T and three Y dwarfs, and new spectra of three Y dwarfs, obtained at Gemini Observatory. We also present previously unpublished photometry taken from HST, ESO, Spitzer and WISE archives of 11 late-type T and 9 Y dwarfs. The near-infrared data are put on to the same photometric system, forming a homogeneous data set for the coolest brown dwarfs. We compare recent models to our photometric and spectroscopic data set. We confirm that non-equilibrium atmospheric chemistry is important for these objects. Non-equilibrium cloud-free models reproduce well the near-infrared spectra and mid-infrared photometry for the warmer Y dwarfs with 425 <= T_eff K <= 450. A small amount of cloud cover may improve the model fits in the near-infrared for the Y dwarfs with 325 <= T_eff K <= 375. Neither cloudy nor cloud-free models reproduce the near-infrared photometry for the T_eff = 250 K Y dwarf W0855. We use the mid-infrared region, where most of the flux originates, to constrain our models of W0855. We find that W0855 likely has a mass of 1.5 - 8 Jupiter masses and an age of 0.3 - 6 Gyr. The Y dwarfs with measured parallaxes are within 20 pc of the Sun and have tangential velocities typical of the thin disk. The metallicities and ages we derive for the sample are generally solar-like. We estimate that the known Y dwarfs are 3 to 20 Jupiter-mass objects with ages of 0.6 to 8.5 Gyr.

• The paper refines mass and age estimates of Y dwarfs using enhanced astrometry, photometry, and near-infrared spectroscopy.
• It applies updated atmospheric models with non-equilibrium chemistry and vertical mixing to match observed spectral features.
• Results indicate Y dwarfs range from 3–20 Jupiter masses and 0.6–8.5 Gyr, underscoring challenges in cloud modeling for cooler objects.

The Y-Type Brown Dwarfs: Estimates of Mass and Age

The research conducted by Leggett et al. presents an extensive paper of the coldest known class of brown dwarfs, the Y dwarfs. Utilizing new astrometric data, homogenized photometry, and near-infrared spectroscopy, this paper aims to refine estimates of the physical properties of Y dwarfs, including their mass and age. The discovery of these extremely cold brown dwarfs has been primarily driven by advances in infrared survey technology, notably through the efforts of the WISE and Spitzer telescopes, allowing the identification of objects with effective temperatures ($T_{\rm eff}$) ranging from 250 to 500 K.

Methodology and Data Collection

The authors have significantly improved the measurements of parallax for select Y dwarfs using the Spitzer Space Telescope, providing more accurate distance and kinematic data. This is complemented by new photometric measurements from telescopes such as the Gemini Observatory and archival data from HST and ESO telescopes. Synthesizing this dataset into a consistent photometric system enables a robust comparison with theoretical models.

Key to the paper are the models of brown dwarf atmospheres, which incorporate updates in chemical opacities for molecules like CH$_4$, H$_2$, and NH$_3$, along with recent non-equilibrium chemistry models influenced by vertical mixing. The authors pay specific attention to models both with and without cloud formations, as clouds can significantly affect the atmospheric and photometric observations of these cool brown dwarfs.

Key Findings

• Estimation of Physical Properties: The paper suggests that Y dwarfs likely range from 3 to 20 Jupiter masses, with ages spanning from 0.6 to 8.5 Gyr. These estimations are derived from comparing models with observational data, taking into account the effects of non-equilibrium chemistry and atmospheric dynamics.
• Atmospheric Modeling: Non-equilibrium chemistry models successfully explain the spectroscopic and photometric characteristics of Y dwarfs with $425 \leq T_{\rm eff} \leq 450$ K. However, for cooler Y dwarfs ($325 \leq T_{\rm eff} \leq 375$ K), the current cloud-free models struggle to fully match the observed photometric data, indicating that some degree of cloud coverage could improve the model fits.
• W0855 Constraints: The analysis of the coolest Y dwarf, WISE J085510.83-071442.5 (W0855), reveals its likely mass ranges between 1.5 and 8 Jupiter masses and age between 0.3 and 6 Gyr. The atmospheric models indicate strong vertical mixing in its atmosphere.

Implications and Future Directions

The expansion of the Y dwarf database with enhanced astrometry and a more unified photometric framework significantly enhances the ability to test and constrain theoretical models of brown dwarf formation and evolution. Understanding the atmospheric conditions of these celestial bodies informs us about the lower temperature limits and chemical complications involved in substellar object atmospheres.

The paper acknowledges the need for further development of atmospheric models, particularly to better capture cloud effects in cooler Y dwarfs. Future instruments with greater sensitivity at longer infrared wavelengths will help address current observational limitations and improve understanding of these intriguing astronomical objects. The potential for variability studies across different spectral bands also presents exciting opportunities for future research.

In summary, the paper by Leggett et al. represents a substantial advancement in characterizing the coldest class of brown dwarfs known to date. By refining the methodologies and models applied to these celestial bodies, the paper lays the groundwork for ongoing and future explorations in both observational and theoretical astrophysics.