2608.10737-Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part I: A sample of Herbig Ae/Be and classical Be 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 I: A sample of Herbig Ae/Be and classical Be stars".

Jocelyn: The paper was written by Arijit Maiti and Mudit K. Srivastava from Physical Research Laboratory and Indian Institute of Technology Gandhinagar.

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

Paper discussion segment 1: IDENT: You're listening to the arXiv astrophysics hour on your local public radio dial — tonight, a two-year spectro-polarimetric campaign that watched young stars change their light.

VERA: Welcome back, everyone. I'm Vera, and with me are Jocelyn and Subrahmanyan. Today's paper is called "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part I: A sample of Herbig Ae/Be and classical Be stars," by Arijit Maiti and Mudit K. Srivastava, from the Physical Research Laboratory and IIT Gandhinagar in India. This one caught my eye because it is exactly the kind of patient observing campaign that doesn't always get the attention it deserves.

JOCELYN: The title tells you a lot: multi-epoch, long-term, variability. They took a spectro-polarimeter called ProtoPol, mounted it on the two-and-a-half-meter telescope at Mount Abu Observatory, and followed twenty-one stars over more than two years. The sample has eleven Herbig Ae/Be stars and ten classical Be stars, and the campaign ran from December 2023 to March 2026 — roughly sixty nights of observations.

VERA: For listeners new to these objects, Herbig Ae/Be stars are young, intermediate-mass stars still surrounded by natal material, while classical Be stars are rapidly rotating B-type stars with a gaseous disk held in a delicate balance around them. Both types have asymmetric circumstellar environments.

JOCELYN: And that asymmetry leaves a stamp on their light, which is what makes spectro-polarimetry such a clever tool.

SUBRAHMANYAN: It is one of the few ways to probe those structures on angular scales of sub-milli-arcseconds, far beyond what direct imaging can reach. What makes this dataset special, though, is the time axis. Multi-epoch spectro-polarimetric observations spanning more than two years are genuinely rare in the literature, and repeated visits tell you whether the geometry is alive and changing, rather than giving you a single frozen snapshot.

VERA: Exactly. And that question — is the scattering environment changing? — gets a clear answer in this paper. For the Herbig stars, yes, dramatically; for the classical Be stars, much less so. The individual stories are quite remarkable, so let's dig into them.

Paper discussion segment 2: JOCELYN: Picking up where we left off with "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part I: A sample of Herbig Ae/Be and classical Be stars," the headline from the summary is this: ten of the eleven Herbig stars showed a detectable spectro-polarimetric signature in at least one epoch, and most of them changed noticeably between the two visits. The classical Be stars, by contrast, were steady — they showed either a depolarization dip across Hα or no line effect at all, and the same behavior in both epochs.

VERA: Some of the individual turnaround stories are genuinely striking. Take MWC 147, a Herbig star that showed a strong polarization increase of two to three percent above the continuum during the first epoch, possibly with a polarization angle flip across the line. Then, more than a year later, the polarization across Hα had essentially vanished.

SUBRAHMANYAN: That is remarkable. And meanwhile HD 58647 did the reverse — almost nothing in the first epoch, then a strong signal peaking around four and a half percent across the central absorption in the second.

VERA: Right, the same star, two completely different spectro-polarimetric personalities.

JOCELYN: What strikes me is that the intensity spectra often look broadly similar between the two epochs for many of these stars, while the polarization structure changes completely. That means the physical conditions in the scattering region — the geometry of the disk, the outflow, the inner envelope — are evolving even when the total emission stays roughly constant. A single snapshot would have been misleading.

SUBRAHMANYAN: And even among the steadier classical Be stars, there are small shifts. ζ Tau's continuum polarization dropped from about two and a quarter percent to about one and a half percent, yet the depolarization signature across Hα remained recognizable. So the Be stars are not frozen, but their spectro-polarimetric behavior is far more predictable than the Herbig stars'.

VERA: That contrast is the central result, and it makes sense — Herbig stars have accretion flows, winds, dust, and young disks all acting at once. And this is where the paper starts thinking about what the community should do with these findings, which we'll take up next.

Paper discussion segment 3: VERA: So we've seen the results — the Herbig stars restless, the classical Be stars steady — and now we're looking at what "Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part I: A sample of Herbig Ae/Be and classical Be stars" suggests for the way forward. One of the most practical contributions is a list of sources that are bright enough for spectro-polarimetry with small-aperture telescopes and that are demonstrably variable in nature. That's a curated starting point for deeper studies.

JOCELYN: And that's a real improvement over the previous state of affairs, where you'd have to dig through archival observations to guess which stars might show something interesting. Here every star has been characterized at two epochs, so you can pick your target based on evidence.

SUBRAHMANYAN: There's also a technological message. ProtoPol was built entirely in-house from off-the-shelf optical components, and it achieves polarization errors of a few tenths of a percent on a two-and-a-half-meter telescope — sufficient to detect these line effects. That demonstrates that long-term spectro-polarimetric monitoring does not require a giant facility. The field has often leaned on high-resolution instruments like ESPaDOnS on four-meter-class telescopes, but those are heavily oversubscribed, and scheduling repeated visits is a serious bottleneck.

VERA: The paper is explicit about this. It calls for coordinated, long-term, time-resolved observations to fully characterize how these systems evolve, and it argues that temporal monitoring is essential for disentangling geometric effects from intrinsic variability. If you observe a star once, you cannot tell whether an unusual polarization profile is a permanent feature or a passing phase; two epochs separated by a year or more let you start separating the two.

JOCELYN: And this is just Part I of the story. The same group is preparing a companion paper with results for symbiotic stars and red giants, which are

Paper discussion segment 4: [Vera]

Conclusion: VERA: A two-year spectro-polarimetric campaign with the ProtoPol instrument has shown that Herbig Ae/Be stars shift their Hα polarization dramatically between epochs while classical Be stars stay stubbornly steady. Jocelyn, what do we take away from this?

JOCELYN: I think the biggest takeaway is that a single snapshot of any of these young stars could have easily misled us. Stars like MWC 147 showed a strong polarization increase one year and nothing the next. HD 58647 did the reverse. The intensity spectra sometimes barely moved while the polarization reorganized completely. That proves the scattering geometry around Herbig stars is alive on yearly timescales, driven by some combination of disks, winds, accretion, and dust that we're only just beginning to untangle.

VERA: And the classical Be stars provided the foil — they mostly showed either a simple depolarization dip or nothing, and that behavior repeated across epochs. That contrast isn't a failure of the technique. It's actually the message: mature, rapidly rotating stars with settled equatorial disks have predictable scattering environments, whereas young intermediate-mass stars are still chaotic.

JOCELYN: Right. The paper also acts as a practical resource — a curated list of which sources are variable and bright enough for small telescopes. That lowers the barrier for other groups to follow up. And ProtoPol itself is a proof of concept that you don't need a four-meter telescope to do meaningful spectro-polarimetric monitoring, as long as you're patient and willing to return to the same targets.

VERA: The authors are already doing that. They mention a companion paper covering symbiotic stars and red giants, which should tell us whether these cool, interacting systems show similarly restless polarization. That's a natural next stop on the arXiv — we'll look at that Paper II when it lands.

JOCELYN: For now, this study reminds us that the universe's smallest angular scales — the inner edges of circumstellar disks, the bases of winds — can be probed with nothing more than a clever polarimeter and two years of dedication. No direct imaging needed, just photons and time.

VERA: Well said. That's all for this paper — thanks for listening, and we'll see you next episode when we dive into a companion study on stars that breathe their material out and pull it back in.