2608.10751-Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars

Introduction to the show: ident: Astrophysics Radio.

Vera: Next we'll be talking about the paper "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars".

Jocelyn: The paper was written by Arijit Maiti, Ruchi Pandey, Sube Singh Gurjar and Mudit K. Srivastava from Physical Research Laboratory and Indian Institute of Technology Gandhinagar and Johns Hopkins University and NASA Goddard Space Flight Center.

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

Title: Vera: Welcome back to the show, everyone. Today we're looking at a recent paper from the arXiv — it's called "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars." The team is led by Arijit Maiti at the Physical Research Laboratory in India, with Ruchi Pandey, Sube Singh Gurjar, and Mudit K. Srivastava — an interesting collaboration stretching from Ahmedabad to Johns Hopkins and NASA Goddard.

Jocelyn: And it's Part II for a reason — Part I covered the hot stars, the Herbig Ae/Be and classical Be stars, observed with the same instrument. This is the cooler side of the sample, which is a nice way to complete the story.

Vera: Exactly. The instrument is ProtoPol, a medium-resolution echelle spectro-polarimeter built in-house and mounted on the 2 point 5-meter telescope at Mount Abu in India. It's a new instrument, and this whole campaign grew out of its performance verification phase — when you first put an instrument on a telescope, you have to point it at something, and they pointed it at a lot of things.

Subrahmanyan: What's particularly nice about the title is the phrase "multi-epoch" — they didn't just observe these stars once and call it a day. They went back over 26 months, from March 2024 to May 2026. That's what lets you see variability, which is the real goal here.

Jocelyn: And the science targets — symbiotic stars and red giants — those are the evolved systems where you'd actually expect the polarization to change. Symbiotic stars are binaries, a cool giant paired with a hot compact star like a white dwarf, and their interaction shapes the surrounding gas and dust. Red giants are losing mass, pulsating, building up dusty envelopes.

Vera: Right. And the whole point of spectro-polarimetry is that you can't directly image these environments — they're too small and too far away. So you measure the polarization of the light, which carries information about the geometry of the scattering material. If the envelope were perfectly spherical, the polarization would cancel out. Any net polarization means something is lopsided.

Subrahmanyan: And when you observe the same star again and again, and the polarization changes, you're watching that lopsidedness evolve. That's the promise of this paper — a rare, long-baseline dataset for stars that don't usually get this kind of attention.

Jocelyn: I'm curious how they actually pulled this off — 24 stars over two years with a brand-new instrument. That's the kind of patience astronomy requires but rarely gets credit for.

Vera: We'll get into the details in a bit. But first — what did they actually find? That's up next.

Summary: Vera: So we've established what this paper — "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars" — set out to do. Now let's talk about what they found. The sample was 24 stars: 6 symbiotic systems and 18 red giants, each observed at least twice over those 26 months.

Jocelyn: And the headline result, at least for the symbiotics, is a bit of a surprise. They found that Hα emission in most of the symbiotic stars did not show any notable polarization signature. That's interesting because there's a long-standing debate about what produces the broad Hα wings in these systems — whether it's Raman scattering of Lyman-beta photons in the neutral wind of the giant, or Thomson scattering of Hα photons by electrons.

Vera: Both of those mechanisms should imprint a polarization signature on the line. So when you see no polarization across Hα, it constrains those models. But then you have the Raman-scattered O VI features at 6830 and 7088 Angstroms — those did show polarization in the systems where they're present, like AG Dra and Z And. In AG Dra, the Raman features showed a clear polarization enhancement in the first epoch but almost nothing in the second. Between epochs, the scattering geometry changed.

Subrahmanyan: And Z And is the opposite story in a way — the degree of polarization across the Raman features stayed roughly constant, but the polarization angle rotated. That's a beautiful diagnostic because it tells you the scattering region has a different orientation than the continuum-scattering region.

Jocelyn: Then there's T CrB, the recurrent nova everyone is watching because it's expected to go into outburst around 2025-2026. They caught its Hα profile flipping between single-peaked and double-peaked across four epochs, and the continuum polarization climbed from about 0 point 5 percent to 1 point 1 percent. That increase suggests the system is developing intrinsic polarization as it gets more active.

Vera: And UV Aur — a carbon Mira — showed something rare: a polarization enhancement across Hα itself, growing from about 0 point 5 percent above the continuum in the first epoch to 1 point 0 percent in the second. That's one of the few clear Hα polarization detections in the sample, and it's worth following up.

Subrahmanyan: For the red giants, the story is about the continuum. All 18 of them showed measurable polarization, which already tells you that asymmetries are common in their extended atmospheres and circumstellar envelopes. And several — LQ Her, Omega Vir, ST UMa, SW Vir, U Her, X Her — showed dramatic changes between epochs. X Her went from 2 to 3 percent polarization down to essentially zero. That's a completely different scattering environment.

Jocelyn: That's the kind of variability that would be invisible in a single snapshot. The observations span more than two years, which makes this one of the rare datasets able to catch these changes.

Vera: And that leads to the question of what comes next — what do the authors suggest we do with all this? We'll get to that in a moment.

Improvements: Vera: We're back with the paper "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars." We've covered the findings — now the question is, where does this leave the field? What do the authors say we should do better?

Jocelyn: They're pretty explicit about it at the end. The key sentence is that future observations with higher cadence, combined with information on orbital and pulsation phases, will be crucial for disentangling the interstellar, instrumental, and intrinsic polarization components. That's the big caveat of this whole study — they intentionally did not correct for interstellar polarization or instrumental polarization, because they were focused on relative changes between epochs.

Subrahmanyan: And that's a fair strategy, but it means the absolute polarization values have to be read with caution. The interstellar component should stay constant across epochs, so if the polarization changes, that's intrinsic to the star. But if you want to know the true level of polarization, you need to model and subtract those constant contributions.

Vera: The other improvement they highlight is timing. For the symbiotic stars, polarization is expected to vary with orbital phase — as the binary components move around each other, the scattering geometry changes. They mention specifically that a higher-cadence campaign on RW Hya over different orbital phases, from conjunction to quadrature, would be useful for testing phase-dependent variability. And for T CrB, well, the system is expected to go into outburst, and they say it will be thoroughly monitored spectro-polarimetrically with ProtoPol through that event.

Jocelyn: There's also a practical point about the instrument itself. ProtoPol needs a signal-to-noise ratio of roughly 230 to 700 per spectral resolution element to reach the 0 point 1 to 0 point 3 percent polarization uncertainty that these studies demand. That's why they implemented an adaptive binning scheme — they widen the spectral bins until the signal-to-noise target is reached, sacrificing spectral resolution for precision. It's a reminder that spectro-polarimetry is photon-hungry, as the paper says.

Subrahmanyan: And for UV Aur, they've already planned follow-up to explore the Hα polarization variability. That's one of the few systems where the line polarization is actually detected, so it becomes a natural target for deeper study.

Vera: The bigger picture is that this paper is establishing a methodology — a rare multi-epoch dataset that others can build on. The next step is combining these measurements with orbital and pulsation phases, which requires knowing the periods well and planning observations accordingly. That's an investment, but it's what you need to actually model the scattering geometries.

Jocelyn: And it connects to a broader point: these are mostly faint, slow-changing systems that don't get the attention of, say, supernovae or gamma-ray bursts. But they're telling us something fundamental about how stars lose mass and die.

Vera: Exactly. And speaking of what motivates the study — the opening pages lay out the physics beautifully. Let's go back to the beginning of the paper and look at how they frame it all.

First page: Vera: We've been discussing "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars" — but let's now step back to the first page of the paper, where the abstract and introduction set the stage. The key sentence is that measuring polarization and its temporal variability provides a powerful probe of small-scale circumstellar environments — on spatial scales otherwise inaccessible to direct imaging.

Jocelyn: And the physics is elegant. When light scatters off dust grains, molecules, atoms, or free electrons, it becomes polarized. But if the scattering region is spherically symmetric around the star, all those polarization vectors cancel out. So any net polarization you detect is a direct fingerprint of asymmetry. A disk, a bipolar outflow, a clumpy wind, an interaction region between binary stars — they all leave a trace in the polarization.

Subrahmanyan: That's why evolved stars are such ideal targets. Red giants and symbiotic stars are surrounded by material produced by stellar winds, dust formation, pulsation-driven mass loss, and binary interaction. The paper mentions that the composition, optical depth, and distribution of the scattering material all affect the measured polarization. So you're not just detecting the asymmetry — you're getting constraints on what the material is and where it sits.

Vera: The first page also introduces the key spectral features that make symbiotic stars so interesting. About half of all symbiotics show those emission features at 6830 and 7088 Angstroms — first documented in 1980 by D. A. Allen, but later understood to be produced by Raman scattering of O VI resonance photons in the neutral hydrogen wind of the giant star. There's also the Raman scattering of Lyman-beta photons, which produces the broad Hα wings you see in many symbiotics.

Jocelyn: And those are exactly the features you can test with spectro-polarimetry. Raman scattering should imprint a measurable polarization, so when you see polarization across the Raman features — like they did in AG Dra and Z And — it confirms the scattering interpretation. When you don't see it — like across Hα in most of the sample — it constrains the models.

Subrahmanyan: There's one more mechanism worth naming — Thomson scattering of Hα photons by electrons. That's been proposed as another origin for Hα polarization, and it's been invoked for the symbiotic star BI Crucis. The paper keeps both options on the table.

Vera: And the instrument itself — ProtoPol — is described on the first page too. It covers the entire visible range from 4000 to 9600 Angstroms with a spectral resolution of 0 point 4 to 0 point 75 Angstroms. It's a fully in-house instrument built with off-the-shelf components, which is remarkable for what it's achieving.

Jocelyn: It's also a reminder that this kind of science isn't just about big facilities — a dedicated instrument on a 2 point 5-meter telescope, built in-house, can produce datasets that the broader community cares about. That's the underdog story here.

Vera: And with that, let's wrap up our discussion of this paper and think about what it all means.

Conclusion: Vera: Time for us to say goodbye to "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part II: A sample of symbiotic and red-giant stars." Let's take stock of what we've learned.

Jocelyn: This was a 26-month campaign with the ProtoPol instrument on the PRL 2 point 5-meter telescope, catching 6 symbiotic stars and 18 red giants across multiple epochs. The main takeaway is that continuum polarization is common among evolved cool giants — all 18 red giants showed measurable polarization, and several showed strong changes between epochs. LQ Her, Omega Vir, ST UMa, SW Vir, U Her, and X Her are the names to remember for variability.

Subrahmanyan: For the symbiotics, the Raman-scattered O VI features at 6830 and 7088 Angstroms remain the most reliable polarization probes — AG Dra showed epoch-to-epoch changes, Z And showed a rotation in the polarization angle. Meanwhile Hα was mostly quiet, which is itself a constraint on the scattering mechanisms. And T CrB is the one to watch: its continuum polarization jumped from about 0 point 5 to 1 point 1 percent as it prepares for a likely outburst.

Vera: The authors emphasize that this is one of the rare multi-epoch spectro-polarimetric datasets spanning more than two years. It establishes the diagnostic power of the technique — polarization as an indirect probe of circumstellar geometry that direct imaging can't resolve. And it sets up the next steps: higher cadence observations, tied to orbital and pulsation phases, to disentangle the interstellar, instrumental, and intrinsic contributions.

Jocelyn: It also proves the instrument — ProtoPol, built in-house with off-the-shelf components, has the stability and precision to track these small polarization changes at the level of a few tenths of a percent. That's no small achievement.

Subrahmanyan: And it leaves us with an open question that's always worth sitting with: if so many red giants show asymmetric envelopes, what does that mean for how mass loss actually happens in the late stages of stellar evolution? The deviations from spherical symmetry seem to be the rule, not the exception.

Vera: With that, we bid farewell to this paper and its contributions to our picture of evolved stars. Stay tuned — the next episode will bring a fresh paper from the arXiv, and we'll dig into whatever the universe has in store for us next.

Jocelyn: Thanks for listening, everyone. Keep looking up.