Affiliation
American Association of Variable Star Observers (AAVSO)
Fri, 12/22/2023 - 22:31
Hello,
When checking in VSX some information about the star CW Cru, one of the targets I monitored recently, I noticed that it is classified as belonging to the "em" spectral class. Is there an "em" spectral class?
Simbad reports it in the B0IIIe C spectral class (Aidelman et al., 2018, Astronomy and Astrophysics, volume 610A, 30-30).
If anyone could comment, please.
Thanks.
Ari
The spectral classification listed in SIMBAD for CW Crucis is B0IIIe. The C is a quality flag and is not part of the spectral designation itself, and strictly the "e" isn't either. It is a qualifier. The classification means that the star is a B0 giant (III) whose spectrum contains emission lines (e). Sometimes "em" is used instead, which also indicates the presence of emission features in the spectrum.
My questioning arose from the apparent discrepancy between the annotation in VSX and the record in Simbad.
With your explanation, the issue made sense to me.
Be stars, or GCAS in AAVSO's variability type notation, is what currently draws my interest but the "em" designation used by VSX regarding CW Cru was new to me. B0IIIe, in Simbad's notation, is more informative.
Thank you for clarifying the subject.
Ari
Ari, you have posted the same question both to the Spectroscopy and the VSX forums. You are getting different replies in each forum. I recommend posting to one forum only to avoid this.
Let's keep the discussion here.
Harold gave you the right answer.
Some comments.
Not all Be stars are GCAS. GCAS are those showing light outbursts.
About B0IIIe being more informative than em, of course. And that's where the observers controbution to VSX becomes important.
Any star can be revised if there is information that improves what we currently show in the VSX record.
I recommend reading this post, that highlights these facts:
https://www.aavso.org/vsx-do-it-yourself
In the case of the star you found, you can revise the spectral type field and provide a reference to the newly added information.
The reference should be in the same format as those listed in the FAQ page.
SIMBAD will link to the relevant reference. You will only have to add the reference name and the bibcode.
Then add "Spectral type from [bibcode]" in the revision comment.
If you don't want to go deeper and do other kind of revisions to this star, that's it.
You can try it!
Cheers,
Sebastian
Sebastian,
Nice to know about the DIY VSX revision possibility and the meaning of the "em" designation.
I'll be working on it, thank you.
Ari.
To some extent I am responsible for the "em" spectral type in VSX. I maintain (or try) a large file of spectral classifications collected from the literature, which is available for query through the CDS VizieR catalogue-query utility as item 'B/mk'. At some point Sebastian or others grabbed that type from the file, I think by matching long lists of variable stars with that file among others.
There are numerous spectral surveys published where folks have used (usually) an objective-prism on a Schmidt camera to hunt for stars specifically with H-alpha emission. They give no traditional temperature type or MK classification, but indicate only that there's the emission. So I have indicated those in my file simply as 'em', and that happens to be one grabbed in VSX data-trawl. If you look in VizieR you'll see that there are four entries for the star, including two where a detailed type is given along with some additional remarks. One recent type has been mentioned not listed there yet. The B/mk file has the advantage that the standard 19-digit bibcode is shown as a hot link, so you can see the source paper from which the type was obtained. The VizieR output has a link to SIMBAD itself as well.
I have just looked at the GAIA3 spectrum for the star. It confirms the temperature-type of B0 or B1 and the strong H-alpha emission shows clearly. The GAIA spectra have rather low spectral resolution (R ~20), so you can't get a reliable luminosity class for B stars from the spectrum alone.
\Brian
Dear Brian,
I feel honored to have your comments on the thread I initiated out of my misunderstanding of the "em" notation.
You added to my learning about the subject by recalling its history.
Being at 20º S, 44º W, I dedicate most of my interests to the targets in the Southern skies.
Thank you!
Ari
CW Cru is listed as a classical Be star in the BeSS database
http://basebe.obspm.fr/basebe/BeSS/visutype.php?type=objet&objet=CW%20Cru
There are three R~1000 spectra in the database by Terry Bohlsen in 2013,2014,2019 all showing H alpha and beta in emission.
Cheers
Robin
The strength of the emission has been effectively constant over that period. The H alpha emission line profile shows no obvious structure at this resolution. The FWHM (after correcting for the spectrum resolution) is ~10 Angstrom