Detectability of resolved hydrogen lines from the accretion shock at gas giants and their CPDs

summary

The gist

As a diligent researcher, I have analyzed both your provided text (A) and the structural pattern in (B) relative to the source paper: *"Detectability of resolved hydrogen lines from the accretion

In short

The episode discusses a paper on detecting resolved hydrogen lines from accretion shocks at gas giants using next-generation instruments like METIS on the ELT. Hosts explore how these lines reveal planet growth, how line shapes diagnose physics like magnetic fields, and suggest current mass accretion rate estimates might be underestimated.

Key concepts

Accretion Shock: This is the glow produced when gas hits a gas giant or its surrounding disc as the planet is still growing. Researchers are looking for this heat signature to catch planets in the act of forming.
Br$\alpha$ line: This specific spectral line was used in modeling to study accretion shocks. The paper found that if a planet like PDS seventy b is observed, the peak of this emission could be as strong as the planet's own light.
Line Width Diagnostics: The shape of the hydrogen line, specifically its core versus its wings, acts as a diagnostic tool. A narrow core suggests slower gas movement (about thirty to forty km/s), which helps distinguish between simple shocks and magnetospheric accretion.

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This episode discusses

The paper

Detectability of resolved hydrogen lines from the accretion shock at gas giants and their CPDs · Read on arXiv

Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany · Fakultät für Physik, Universität Duisburg–Essen, Lotharstraße 1, 47057 Duisburg, Germany · Division of Space Research & Planetary Sciences, Physics Institute, University of Bern, Sidlerstr. 5, 3012 Bern, Switzerland · School of Physics and Astronomy, Sun Yat-sen University, Guangdong 519082, People’s Republic of China · Kapteyn Astronomical Institute, University of Groningen, PO Box 80, 9700 AV Groningen, The Netherlands

Fewer gas giants have been caught in their accretion phase than mature ones are known. Extremely Large Telescope (ELT) instruments will have a higher sensitivity and a smaller inner working angle than tools up to now, which should increase search yields. We examine what METIS, the first-generation ELT spectrograph with R=1e5, can reveal about accreting gas giants. We focus on the accessible hydrogen recombination lines, mainly Brackett alpha and Pfund-series lines. Our approach is general but we take PDS70b as a fiducial case. It is similar to WISPIT2b. To calculate high-resolution line profiles, we combine a semianalytical multi-D description of the flow onto an accreting planet and its circumplanetary disc (CPD) with local non-LTE shock-emission models. We assume the limiting scenario of no extinction, appropriate for gas giants in gaps, and negligible contribution from magnetospheric accretion. We use simulated detector sensitivities to compute needed observing times. Both the planet- and the CPD-surface shocks contribute to the line, which has a Gaussian core but wider, asymmetrical wings. The line is much narrower than the free-fall velocity, and in fact has a nearly constant FWHM=30--40 km/s at low densities. For our fiducial accretion rate onto PDS70b, the Br-a line peak excess is as strong as the photospheric continuum, modulated mostly by H2O features. At Br-a, already the continuum of PDS 70 b yields a per-bin S/N=12 in 4h. With ProDiMo, we estimate the CPD not to hinder the detection of the line emission. The peak excess should require only 10 min to reach S/N=3. For pure shock emission, the line shape is barely sensitive to the planetary or system parameters. A complex profile would indicate that magnetospheric accretion contributes significantly. The high spectral resolution of METIS will help reveal line shapes even of faint accretors with great fidelity.

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Next we'll be talking about the paper "Detectability of resolved hydrogen lines from the accretion shock at gas giants and their CPDs".

Jocelyn: The paper was written by Gabriel-Dominique Marleau, Thomas Henning, Roy van Boekel, Myriam Benisty, Yuhiko Aoyama et al. from Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany and Fakultät für Physik, Universität Duisburg–Essen, Lotharstraße 1, 47057 Duisburg, Germany and Division of Space Research & Planetary Sciences, Physics Institute, University of Bern, Sidlerstr. 5, 3012 Bern, Switzerland and School of Physics and Astronomy, Sun Yat-sen University, Guangdong 519082, People’s Republic of China and Kapteyn Astronomical Institute, University of Groningen, PO Box 80, 9700 AV Groningen, The Netherlands.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Paper discussion segment 1: Vera: We’re starting our show with a paper titled "Detectability of resolved hydrogen lines from the accretion shock at gas giants and their CPDs." Jocelyn, this title is basically telling us exactly what the researchers are hunting for with the new ELT instruments.

Jocelyn: It sounds like they are looking for the specific glow produced when a giant planet is still growing. They're focusing on these hydrogen lines coming from the shock where gas hits the planet or its surrounding disc.

Vera: Right, and that's a huge deal because we see plenty of old, stable gas giants, but seeing them while they are actually feeding is much harder.

Jocelyn: So, instead of just seeing a cold ball of gas, they want to see the heat from the impact itself?

Vera: Exactly. They're looking at these "accretion shocks" to catch planets in the act of forming.

Subrahmanyan: This is vital because if we can watch this process, we move from just taking snapshots of finished planets to actually seeing how they assemble. The paper focuses on how well our next generation of telescopes can resolve those specific signatures.

Jocelyn: Does that mean we've been missing them because our current tech just isn't sensitive enough?

Subrahmanyan: In many cases, yes, because the signal from a single planet is incredibly faint compared to the surrounding dust and gas in the protoplanetary disc. We need high spectral resolution to separate the planet's specific "fingerprint" from all that background noise.

Vera: Which leads us directly into how they actually modeled these signals to see if we can even do it.

Paper discussion segment 2: Vera: Now that we know what they're looking for, let’s talk about how they modeled it, specifically using the Brα line. They used a case study of PDS seventy b to see if the signal would be strong enough to actually stand out.

Jocelyn: I noticed they mention the Brα line is surprisingly potent. If we look at a planet like PDS seventy b, the peak of that emission might be as strong as the planet's own light.

Vera: And the modeling shows these lines have a very specific shape—a Gaussian core with these wider, asymmetrical wings.

Jocelyn: Wait, if the wings are wide but the core is narrow, what does that tell us about how fast the gas is moving?

Vera: The paper says the line width stays pretty constant at about thirty to forty kilometers per second, which is much slower than the actual free-fall velocity of the gas.

Subrahmanyan: That’s a fascinating result because it suggests we shouldn't expect these lines to be super broad even when the gas is falling in very fast. It also means that if we do see a really broad or complex profile, we might be looking at magnetospheric accretion rather than just a simple shock.

Jocelyn: So the shape of the line acts like a diagnostic tool for the physics happening at the planet's surface?

Subrahmanyan: Precisely, it tells us about the magnetic field and how much gas is actually landing on the planet versus just swirling around in that circumplanetary disc.

Vera: It’s amazing how much information is packed into that one specific spectral line.

Paper discussion segment 3: Vera: We've talked about the physics, but let's look at the actual hardware potential, specifically the METIS instrument on the ELT. The paper makes some huge claims about how much better this will be than what we have now.

Jocelyn: It says METIS is about seventy times more sensitive in the L band than CRIRES+. That's a massive jump for an observer.

Vera: It really is, and it means we could potentially detect the shock peak of a PDS seventy b-like planet in just ten minutes.

Jocelyn: Ten minutes? We usually spend hours on a single target! But there’s a catch in the paper about how they calculate accretion rates, right?

Vera: Yes, they found that current "empirical" methods might be underestimating how much mass is actually reaching the planet.

Subrahmanyan: That's because traditional models assume all gas hits at maximum free-fall velocity, but this research shows the average velocity is lower. This means the actual mass flux hitting a two Jupiter-mass planet could be three to ten times higher than what we've been estimating.

Jocelyn: So our current understanding of how fast planets grow might be fundamentally skewed?

Subrahmanyan: It suggests we might have been undercounting the growth rate because our "rulers"—the empirical models—were calibrated incorrectly.

Vera: There is also a warning in there about water opacity and how different molecular databases can change the predicted profiles.

Conclusion: Vera: This has been an incredible look at "Detectability of resolved hydrogen lines from the accretion shock at gas giants and their CPDs." We've seen that METIS is going to be a game-changer for catching planets in their formative years.

Jocelyn: It’s exciting to think that we might soon move from just knowing planets exist to actually measuring how they grow in real-time.

Subrahmanyan: It really shifts the paradigm from static observations to dynamic, physical characterization of planetary birth.

Vera: We definitely need more work on those water opacities before we start claiming we've mapped every planet's atmosphere perfectly.

Jocelyn: Agreed, but the potential for these hydrogen lines is enormous. Thanks for joining us!

Subrahmanyan: It’s a great time to be an astronomer. See you next time!

Vera: Goodbye everyone! We'll see you after the break with our next paper.

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