astrobites: Daily Paper Summaries 2020

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Don’t change the station! Substellar objects are up next!

Post by bystander » Thu Oct 22, 2020 10:24 pm

Don’t change the station! Substellar objects are up next!
astrobites | Daily Paper Summaries | 2020 Oct 21
Ali Crisp wrote:
What do you get when you have too much mass to form a planet, but not enough mass to form a star? A brown dwarf! First theorized in the 1960s and observed in the 1990s, brown dwarfs (BDs) – a subclass of ultra-cool dwarfs – are substellar objects around 13-80 times the mass of Jupiter (or 10-90 times, depending on who you ask). They are special because, though they are thought to form in a similar manner to stars, they aren’t massive enough to trigger sustained hydrogen fusion in their cores. Instead, they are thought to fuse deuterium or lithium. This means that, unlike our Sun or other stars, they will gradually cool and fade rather than becoming white dwarfs, neutron stars, or black holes.

Despite not being stars, BDs are still self-luminous – meaning they emit energy in the form of light rather than just reflecting it back from a host star like planets do – and therefore can have spectral classifications like stars. Depending on how much light they emit and their temperatures, BDs are classified as either L, T, or Y type. Each class shows different dominant absorption lines, with L dwarfs being water and carbon monoxide-dominated, T dwarfs being methane-dominated, and Y dwarfs potentially being ammonia-dominated.

Like stars, some BDs are known to have strong magnetic fields, and even instances of potential aurorae. In addition to being observable by some optical instruments, this magnetic activity allows some BDs to be detectable in the radio and – if the magnetic field activity is strong enough – X-ray bands. However, radio observations of these objects have previously been performed primarily to follow-up known BDs. The authors of today’s paper use the Low Frequency Array (LOFAR) to make the first direct radio observation of a brown dwarf, BDR 1750+3809. They specifically looked at circularly polarized (CP) radio sources in the LOFAR Two-meter Sky Survey (LoTSS), because known brown dwarfs have highly circularly polarized radio emission. They followed up the LoTSS data with near-infrared observations using the Wide-field Infrared Camera (WIRC) at Palomar, and the NIRI imager at Gemini-North. They also obtained a spectrum using NASA’s Infrared Telescope Facility (IRTF). ...

Direct Radio Discovery of a Cold Brown Dwarf ~ H. K. Vedantham et al
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Spectacular Specular Reflections on Titan

Post by bystander » Thu Oct 22, 2020 10:39 pm

Spectacular Specular Reflections on Titan
astrobites | Daily Paper Summaries | 2020 Oct 22
Anthony Maue wrote:
A specular reflection, like the glint of the sun reflecting off the ocean at sunset, is generally a pretty good indicator of a smooth liquid surface like a calm lake. Although specular reflections observed for Saturn’s moon Titan were initially taken as evidence for seas of liquid hydrocarbons, the Cassini mission discovered that Titan’s lakes and seas were isolated mainly to the poles and absent from the low-latitudes that had reflected so brightly in Earth-based observations. So what other terrains could explain these observations?

First, let’s take a second to consider how these reflections work. Light interacting with a surface will be absorbed, transmitted, and/or reflected. A specular reflection occurs for a mirror-like surface where the beam reflects at an angle that is equal to the incoming angle, or incidence (see Figure 1). A sufficiently rough surface will scatter light diffusely, at a multitude of angles (isotropic at the theoretical extreme). Reality usually lies somewhere between these two endmembers.

Today’s authors reconsider radar observations of Titan that were previously made with the Arecibo Observatory (AO) and Green Bank Telescope (GBT). Measurements were mostly limited to Titan’s southern tropics (~7–27°S) due to the structure of the instrumentation and Titan’s orbital parameters. Each observation emits and receives the echo of a radio wave pulse (with wavelength of 12.6 cm) that is akin to scanning a slit across Titan’s diameter roughly 5150 km long and 14 km wide. This process results in a spectrum, such as the ones shown in Figure 2. The magnitude of the radar power is reported as the normalized radar cross section (NRCS), which is the ratio of the backscattered signal to that expected from perfect isotropic scatter. For this work, the authors focus on the maximum-NRCS, usually found as a peak at the center of the echo spectrum. This max-NRCS value is considered an anomalously specular radar reflection (ASRR) when the value is significantly higher than background noise, indicating a strong deviation from diffuse scattering. ...

The Root of Anomalously Specular Reflections from Solid Surfaces on Saturn’s Moon Titan ~ Jason D. Hofgartner et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor