Talk:Epsilon Cassiopeiae

Unclear and Contradictory

Re Navi: the entry for Gamma Cas says that Grissom named that star "Navi".

Which is it? —Preceding unsigned comment added by 86.145.64.252 (talk) 21:42, 18 November 2008 (UTC)

I'm not seeing a contradiction here. Which what is supposed to be what? Lithopsian (talk) 18:45, 26 February 2017 (UTC)

This article also states the star is both main-sequence and a giant. It can't be both, so which is it? 2601:441:4102:9010:5925:D2A4:84F4:CC8F (talk) 17:52, 26 February 2017 (UTC)

Everything except one statement indicates it is a main sequence star, so I removed the statement that it is a giant. Some old (pre 1950) spectral types gave it as a giant or subgiant, which might make an interesting little story if someone wants to dig out the references. Lithopsian (talk) 18:44, 26 February 2017 (UTC)

Lithopsian I bow to your expertise here. What is the true parameter(s) defining selection of B2V and B2III stars then? Looking at the article, the selection of physical parameters ultimately depends on that simple fundamental question? Discussions of B-type stars Class B and B-type main-sequence star BIII are not even mentioned. Itchycoocoo (talk) 09:34, 30 August 2023 (UTC)

Not sure I understand the question. B2III would be a giant. Nobody really thinks this is a giant star any more, not by the spectrum, not by its physical parameters, not in evolutionary terms. The article doesn't mention this spectral type and doesn't mention anything about giant stars. Gaia DR3 models, admittedly not ideal for this star or this type of star but just a quick thing to look up, show it as being on the hook at the end of the main sequence life where core fusion has actually stopped, the core is contracting, and the surface temperature increases until shell fusion begins and the star becomes a subgiant. I'd want to see confirmation from other models, preferably a study specific to this star, before claiming all that as fact. The age of 15 million years showing in the starbox suggests it isn't quite that evolved. Lithopsian (talk) 14:51, 30 August 2023 (UTC)

"Not sure I understand the question. B2III would be a giant. Nobody really thinks this is a giant star any more, not by the spectrum, not by its physical parameters, not in evolutionary terms."

Lithopsian That's the point though. Not having this stated under any topic supports anything relevant to this B2V / B2III star as requested by the IP. As WP:OR edits are an issue here it hard to fix. The best premise of B stars calculations are Nieva, M-F-., “Temperature, Gravity and Bolometric Correction Scale for Non-Supergiant OB Stars.”, arXiv, 2012.0928 (2012) [1] or original [2] As stated B2III (and B2IV stars) exist. Data given in the article seem to be picked by both assuming it's an B2III and B2V. The question is how to fix it. Itchycoocoo (talk) 09:43, 31 August 2023 (UTC)

I still don't understand your question. It isn't a giant. Why are you stuck on the idea that it is? Are you deciding it yourself, or looking at sources from the last century blindly copying spectral types published in the first half of the last century? Lithopsian (talk) 13:42, 31 August 2023 (UTC)

Lithopsian Nobody has denied this star is placed in the main sequence basket. What is being questioned is exactly how are B-stars classified as main sequence or as giants.

Available Wikipedia sources say little to nothing about that. For the general readership it is confusing. Your first response to me is wise and would be considered as likely correct. However, Epsilon Cas (and the associated B-class star articles) has not been supported by needed references or citations.

You state: “Everything except one statement indicates it is a main sequence star, so I removed the statement that it is a giant.” I just thought you knew why.

Reasoning as to why is now plainly obvious.

It seems that the broader reason why B-type stars are so different is due to the problem that they are a closely knitted group on the H-R diagram. Lower spectral classes are likely more easily differentiated based on luminosity and colour.

According to Garrison & Gray (1994), “The MK classification system for the B type stars has undergone some revision and refinement by Morgan. This refined classification system is encapsulated in the system of MK dagger standards (Morgan & Keenan 1973) and further elaborated in the spectral atlas of Morgan et al. (1978).”

ε Cas is a peculiar B-type (B3 Bp) as determined by its spectrum. Luminosity classes show small differences indicated by unexpected deviations with narrow β (δβ) when using Strömgren photometry. (The reason why this is happening is still to be fully determined.) e.g. The basis of the change from III to V luminosity class is the Strömgren photometry observations made in 1968.

After about 1990, the B-types have their luminosity classes depend on Si/He line ratios, Stark effects in certain He I lines, and to a lesser extent, the CNO line structure. Determined ratios are mainly between the two selected lines Si III at 455.2nm & He II at 438.7nm, with ratio differences gradually increasing between luminosity B1 V to B Ia. This ratio is more pronounced with stars having rich OII spectra in B Ia/Ia+ and distinguished by the [Fe II] emission line at 447.1 nm. that exists on either side of the Hγ line at 434.047 nm. High ratios between He lines at 414.4 nm. & 412.1 nm. define the spectral classes of B2 (or B3) stars.

Standard luminosity classes for B2 stars, for example, are HD 141318 B2 II, Gamma Orionis B2III, Gamma Pegasi B2 IV & HD 42401 for B2 V.

Once the luminosity class is selected, then Teff, mass, luminosity, age, etc. can be determined.

Itchycoocoo (talk) 06:08, 1 September 2023 (UTC)

Well, here is perhaps not the best place to discuss such detailed explanations of the general case of determining spectral classes. There is Stellar classification which goes into some detail, but not really this much. There are also articles such as Blue giant and B-type main sequence star, as well as Blue supergiant. You might consider adding some more detail there, although some of it is already included.

I would disagree with your claim that physical parameters are only determined once the luminosity class is known: the assignment of a luminosity class, followed by assumptions about what such a star should look like, is becoming largely (not completely) obsolete. Physical parameters are increasingly determined more directly from observations of distance, temperature (determined from spectral fits or colour indices rather than a one-dimensional spectral class), surface gravity, and sometimes angular radii and binary orbits. Where distance in particular is not known, assumptions may be made about how luminous or how large a giant, for example, should be. We might refer briefly to the methods used to determine the particular physical properties of this star, rather than just list them, but strictly describing what published sources say rather than working it out for ourselves. The current values in starbox detail is a bit of a mix, some from Gaia DR3, some from older models. I added metallicity from Gaia DR3; it tends to give rather extreme values, but I couldn't find another source for [Fe/h], probably because of the spectral peculiarities. Lithopsian (talk) 13:48, 1 September 2023 (UTC)

"I would disagree with your claim that physical parameters are only determined once the luminosity class is known: the assignment of a luminosity class, followed by assumptions about what such a star should look like, is becoming largely (not completely) obsolete."

I don't think I've ever said: "...physical parameters are only determined once the luminosity class is known." It's just the first prerequisite before delving into individual B-type star's astrophysical parameters. It's just convention.

Again reading papers like: Nieva, M-F-., “Temperature, Gravity and Bolometric Correction Scale for Non-Supergiant OB Stars.”, arXiv, 2012.0928 (2012) [stated before] or Firnstein, M., Przybilla, N., “Quantitative Spectroscopy of Galactic BA-type Supergiants.”, arXiv, 1207.0308v1 (2012)[https://arxiv.org/pdf/1207.0308v1.pdfTo quote the later reference:

"Empirical spectral-type–Teff relations provide important starting points for all kinds of stellar studies and for quantitative spectroscopy in particular, and they are therefore an essential part of the reference literature on stellar properties. Our high precision/ high-accuracy dataset facilitates to reassess the existing knowledge in the BA-type supergiant regime in view of improved models and analysis techniques."

However, the most recent technical summary supports my broader comment, Zorec, J., "BCD Spectrophotometry and Rotation of Active B-Type Stars: Theory and Observations.", Galaxy, 11(2), 54 (2023)[3]. See Section 2.5.2. The Apparent HR Diagram of Be Stars & Figure 4.

"...where l in Å is the value of λ₁ –3700 Å, determined at D= 0.2 dex and can be considered as the BCD continuous luminosity class parameter." An example of determining spectral spectral classification/luminosity classes is HD 45677 - a likely close example to ε Cas.

I do appreciate your last response, but is becoming now too technical to simplify into something useful.

I'd gently caution using Gaia DR3 parallax data as the basis of distance unless the value is qualified with an understanding of the corrections. This still applies for star magnitude >3.5. 09:00, 2 September 2023 (UTC) Itchycoocoo (talk) 09:00, 2 September 2023 (UTC)

Yes, unusual to see Gaia parallax at all for a star this bright. The very large (for Gaia) margin of error indicates its unreliability without looking further. The margin of error in DR3 is about twice as large, although Simbad does prefer it to Hipparcos. The Hipparcos parallax margin of error is nearly twice as large again, although there may be fewer systematic errors involved. Not really an issues for Wikipedia, we really shouldn't be calculating anything based on the distance, and if a peer-reviewed source does so then we can assume adequate thought has been given to it (in context, which may be just a large semi-automated database) and we can use it as a reference. Lithopsian (talk) 16:50, 3 September 2023 (UTC)

Distance Gaia DR3

Lithopsian This edit [4] says DR3 parallax is 7.0037±0.1599, but this does not equal the distance given in parsecs or light years. I.e. 470±10 pc. It should be 142.78±8.18 pc.

If true, SIMBAD says the error is ±0.2487 mas NOT 0.1599 mas, and that distance is 142.782±5.0701 pc.?

I also know that DR3 parallax results need corrections based on proper motion and the mean BP/RP spectra using six-parameter (6-p) astrometry. Where d=1000/@pi is not now correct, and DR2 and EDR3 results are different from DR3 results.

Even the given reference says: "3.3. Systematic errors. The parallax bias (and the proper motion systematic errors) varies as a function of magnitude, colour, and celestial position. This is extensively investigated in Lindegren et al. (2021a) and a recipe for correcting the parallaxes is given. The systematic errors in the broad band photometry are described in Riello et al. (2021)."

Because of the star's brightness, the errors are large, and as said in the same link: "GSP-Phot distance estimates are underestimated for sources with large parallax errors because of an extinction prior that then dominates the distance inference." See Figure 2 in the linked document, showing the errors involved with bright stars. According to the correct solution value, this distance is 116.13 pc. using ESASky 5.11 [5]for DR3.

Quote 7.0037±0.2487 without the correction is probably better, and use 142.78±8.18 pc.

• I note the same type of mistakes duplicated on other stars like Mu Ursae Majoris.

Itchycoocoo (talk) 07:00, 30 August 2023 (UTC)

Stated article radial velocity is (Rv)=−8.1[5] km/s Gaia DR3 is -1.9±6.09 km/s. If you using Gaia DR3 as π and proper motions, then why use 1999s outdated FK6? All data is just incoherent numbers without any relevant context? Itchycoocoo (talk) 08:04, 30 August 2023 (UTC)