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RW Cephei great dimming

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#151 yuzameh

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Posted 19 January 2023 - 04:54 PM

I've just noticed, well I'd seen it before but it has only just registered in the brain, that there are 2MASS, WISE, AKARI IRC, MSX6C, AKARI FIS and IRAS fluxes available for this object.  Granted it is from multiple epochs and that the thing is variable but it means anybody interested enough could generate a SEDs plot with multiple flux measures from 1.25 in the NIR, with quite a few other passbands in the NIR, through the MIR, and even well into the FIR, with some very little wavelength redundancy between surveys, and dig out some modelling papers from somewhere to assess if there is any evidence of long term circumstellar matter.  It's finding a decent and relevant paper that's the hard part.



#152 robin_astro

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Posted 19 January 2023 - 07:01 PM

Exactly how big are the errors in the RVs? There seems to be a rapid upward trend in the latest values (though it's hard to tell since it's so zoomed out) but I'm unsure if the errors make the change significant.

The THO (Three Hills Observatory) points in orange are my recent measurements. The scatter is within the uncertainties. There has been no detectable changes in either the low or high resolution spectra above the uncertainties over the past month.

 

The uncertainty in my radial velocity measurements here is ~+-2km/s  (0.04 Angstrom or ~1/10 of the spectrograph resolution), limited by the spectrograph stability. (The measurement is made by cross correlating a wide region of the spectrum against a template spectrum and  the absolute accuracy is checked by recording reference stars with known RV )

 

In spectra where I can see the telluric lines and use them to correct any small calibration offsets, I can improve this to ~1km/s (Not possible with this target because of the dense number of stellar lines) but then I am limited by variations in the positioning of the star on the slit which can very slightly distort the line profile shape. (We are talking here about detecting shifts which are a small fraction of the width of the spectral line.) 

 

Beyond that you need a fibre fed spectrograph mounted off the telescope like the ELODIE spectrograph (famous for discovering exoplanets). which can get down to a few m/sec precision so the ~5km/s dip  seen during the 20 month ELODIE run 2003-2005 will be real.

 

Cheers

Robin



#153 VY Canis Majoris

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Posted 20 January 2023 - 12:48 AM

I've just noticed, well I'd seen it before but it has only just registered in the brain, that there are 2MASS, WISE, AKARI IRC, MSX6C, AKARI FIS and IRAS fluxes available for this object.  Granted it is from multiple epochs and that the thing is variable but it means anybody interested enough could generate a SEDs plot with multiple flux measures from 1.25 in the NIR, with quite a few other passbands in the NIR, through the MIR, and even well into the FIR, with some very little wavelength redundancy between surveys, and dig out some modelling papers from somewhere to assess if there is any evidence of long term circumstellar matter.  It's finding a decent and relevant paper that's the hard part.

Well, for one the mass loss has been estimated already in a 1998 paper to be on the order of 10^-5 Msol/yr which could be some indication already. I also remember finding a spectrum using IRSA that went well into the mid or far infrared we could perhaps use.

 

Edit: I found the spectra: https://irsa.ipac.ca...s_spsc.tbl.html

 

57401207_sws.png

57401207_pws.png


Edited by VY Canis Majoris, 20 January 2023 - 02:52 AM.


#154 yuzameh

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Posted 20 January 2023 - 09:56 AM

Well, for one the mass loss has been estimated already in a 1998 paper to be on the order of 10^-5 Msol/yr which could be some indication already. I also remember finding a spectrum using IRSA that went well into the mid or far infrared we could perhaps use.

 

Edit: I found the spectra: https://irsa.ipac.ca...s_spsc.tbl.html

 

 

Well, there you go, I think this might have been mentioned before, but you can plainly see the "biggest" feature is the "silicon bump" or "silica bump" or "silicate bump" around the 10 micron mark.  That's certainly circumstellar.  Silica, or Silicon Dioxide if you prefer, is better known by names such as quartz or sand, however these particles will likely be a lot finer than sand, note they're in emission not absorption too which means (I think) the energy they receive from the star is preferentially emitted in the infrared when they de-excite.  I've no idea whether it's due to orbitals or rovibrational or whatever.  Rovibrational is usually the case for gas and organic molecules but I've no idea about inorganic solids in the infrared, as stuff done on asteroid surfaces is reflection spectroscopy which I think is something different altogether.

 

A potted summary : https://www.universe...d-spectroscopy/

 

Which reminds me, I must look up the absoprtion route for TiO and VO etc, I can't remember what the mechanism for that is either.

 

Meanwhile the way a SED trails off at the longer wavelengths is supposed to be informative with respect to presence or not of discs or toroids etc, but that's one of those "I've heard of it" things, I've only a vague impression why and whether the profile should decline steadily, instead of abruptly for no torus, or actually go back up again at longer wavelengths.  The professionals can actually model dust temperature from these things.


Edited by yuzameh, 20 January 2023 - 09:57 AM.


#155 VY Canis Majoris

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Posted 21 January 2023 - 03:10 PM

There seem to be some major differences between the dimmings of Rho Cas and RW Cep. While the dimmings of Rho Cas seem to last around a year at most, the most recent one of RW Cep has lasted for more than around 3 years and previous dimmings have lasted around 5-6 years in total. Rho Cas also appears to get H-alpha emission right before the dimmings, rather than during it.

 

Now, the radial velocity appears to evolve oppositely (becoming smaller instead of larger), but this can be explained by differences discussed previously. I am not however sure how those differences change the way H-alpha appears to evolve.



#156 robin_astro

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Posted 21 January 2023 - 04:00 PM

 

Now I need to look up the latest lightcurve and see if the profile is consistent between observersion even if the so called V photometry is all over the place.

To see the underlying trend without the inter-observer scatter, just highlight Wolfgang Vollmann's observations to see the trend  He has been observing it consistently through the dimming and  issued the ATel alert. His results currently show a pause from the end of November 2022 which tallies with the lack of change seen in the spectrum currently. We will probably need a few more weeks/months data to know if it is the bottom of the curve or just another pulsation wiggle

 

 

RW_Cep_Vmag_20230121.png

 

 

Cheers

Robin


Edited by robin_astro, 21 January 2023 - 04:02 PM.

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#157 yuzameh

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Posted 21 January 2023 - 11:43 PM

There seem to be some major differences between the dimmings of Rho Cas and RW Cep. While the dimmings of Rho Cas seem to last around a year at most, the most recent one of RW Cep has lasted for more than around 3 years and previous dimmings have lasted around 5-6 years in total. Rho Cas also appears to get H-alpha emission right before the dimmings, rather than during it.

 

Now, the radial velocity appears to evolve oppositely (becoming smaller instead of larger), but this can be explained by differences discussed previously. I am not however sure how those differences change the way H-alpha appears to evolve.

If we really talking about lumps of muck being randomly chucked out there's never going to be the same circumstances twice as the size, shape, direction, mass, density, composition, outflow acceleration and umpteen other things are going to be quite variable.

 

You won't even see the same situation in the same object repeat in the same way.  For analogous situations of muck chucker-outers stare at long term RCB star lightcurves or DYPER's lightcurve or some UXORs, especially UX Ori the type star.  Totally different kinds of stars which fade due to mass ejection, never the same twice, onset and recovery can follow a general pattern, for example steep fades and slow recoveries which fits a dense and more massive outburst of expanding opaque material.  Many things can cause Halpha emission too, and the profiles of Halpha (peaks, troughs, peaks and troughs especially of different heights, the V and R peak ends, the twin peaks with an abosrption trough, the so-called "P Cygni" profiles, the peak width, can all say things about the type of expansion, speed, possibly density, and give evidence of other materials especially when there are absorption cores, temperature, geometry and vector of any outburst if lucky, but they can be more symptomatic than diagnostic, as the stuff still can be slung out in many varied ways.  You'll find no twins.

 

From the graph r_a provides in the previous post you can get the suggestion that the current more defined fade, because before that it could just have been a pulsation stage although the mean magnitude does appear to have dipped, but the current fade so far doesn't appear to have a hint of pulsational wiggle.

 

There was a thread on BAAVSS alerts about a Mira recently where some were going on about a Mira with brighter than ever before maximum.  What they neglected to take note of was that it was a brighter than usual minimum following, and consequently the amplitude stayed the same.  With erratic pulsators of low amplitude and no real cyclicity, just a general timescale, of pulsation, only the latest dip looks like an obscuration event to me.  The others could simply be a harmonic effect due to multiple pulsation periods, sometimes called 'long secondary periods', that long period pulsators can have, which can lead to mean mag movement, and actually amplitude increase and decrease, as with alpha Orionis.  The current fade is following a different pattern, is deeper than normal, and of somewhat shorter duration so far, but time will tell.  To me it was no coincidence that alpha Orionis had the bonus situation of deep fade at a time when three cycles were coinciding to give a deeper than usual pulsation stage anyway.  This doesn't seem to be the case for this star.  But one is red supergiant and the other a yellow hypergiant, which isn't the same.



#158 VY Canis Majoris

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Posted 23 January 2023 - 03:49 AM

The dust temperature of RW Cep's infrared excess has been calculated to be 300 K in https://ui.adsabs.ha.....229S/abstract.



#159 yuzameh

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Posted 23 January 2023 - 11:56 AM

The dust temperature of RW Cep's infrared excess has been calculated to be 300 K in https://ui.adsabs.ha.....229S/abstract.

pretty nearly room temperatue



#160 VY Canis Majoris

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Posted 23 January 2023 - 12:00 PM

Looking at the spectrum, there doesn't appear to be considerable circumstellar material besides the silicates. It certainly doesn't look as extreme as something like VY Canis Majoris. I could be wrong though. Maybe I should find spectra of Rho Cas in similar spectral regions to compare.

 

EDIT: After comparing with Rho Cas, the flux appears to become smaller a lot sooner and more abruptly with wavelength than in RW Cep.


Edited by VY Canis Majoris, 23 January 2023 - 12:19 PM.


#161 yuzameh

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Posted 23 January 2023 - 03:58 PM

Looking at the spectrum, there doesn't appear to be considerable circumstellar material besides the silicates. It certainly doesn't look as extreme as something like VY Canis Majoris. I could be wrong though. Maybe I should find spectra of Rho Cas in similar spectral regions to compare.

 

EDIT: After comparing with Rho Cas, the flux appears to become smaller a lot sooner and more abruptly with wavelength than in RW Cep.

All these things are a really good way to learn your astronomy.

 

You can read all the things going, the theory, the books whether legendary, generally formal or popular, the articles online.

 

But take an object, especially one that has some level of atypicality, and intercompare it with others by getting all the data together as possible to do, and checking, and looking up what certain things mean, and you learn tons of stuff.  Practical learning.

 

There is a lot to be said for people getting the hang of the infrared and its spectroscopy, I've been saying that for years.  For star type stars (not black holes nor neutron stars etc where you are likely better off with radio and xrays) infrared is going to reveal a lot, especially about their initial stages and their endtimes.  All this nitpicking about early galaxies and trying to find the start of the universe is all well and good, and interesting to some extent, but truthfully stars are often more fun in astronomy, although not necessarily much to look at for a visual observer unless they're binaries or at a pinch variables.

 

This star seems to be semi-legendary because of its nature in the sense that it is known about and mentiond as a bit of a unique star with some "record breaking" bits and pieces involved, but seems to have missed regular in depth study.  It's not that faint, an all year around target for Northern observatories, but only a handful of people seem to have actively targetted, like the chap robin mentioned the paper about.

 

post-AGB, stars are a big topic with some professionals (although the topic may more often be dealt with as a subset of an interest in all AGB stars to some), albeit as a bit of a niche. AGB stars are in fact it is one of the few niche topics that still has its own newsletter, most other newsletterss have died and gone away.

 

https://www.astro.ke...Bnews/view.html

 

The bias nowadays though is going to the other end of a star's life, the start.  They're doesn't seem to have been much microwave mapping for instance of these objects, although some lovely images have come from ALMA of some AGB stars that throw off rings (spheres possibly, we see images projected upon the plane of the sky).

 

Possibly this start isn't likely thougth of as an AGB or post-AGB.  Though, if you cheated only slightly on definitions, very massive stars like this, and VY CMa for that matter despite it being more dissimilar to RW Cep than RW Cep is to rho Cas, could be said to be born as AGB or post-AGB, and probably have lifespans of ten million years or less.  When I see lifespan I don't necessarily mean full age from start to very finish, I mean their being as they are now.  That's why there are so few around at this time, it is a short lived phase as well as being a rarer kind of star.

 

You'll probably always remember this one even after it is all done and done with and years have passed, and you will certainly absorb the knowledge and information you glean far better than you would have otherwise.  I personally find objects like this far more interesting that all the eruptives that are the fad nowadays, which at the end of the day are all pretty samey.

 

Amateur spectroscopists also add some greater interest to it too as such information did not appear publicly until formal papers, if any, eventually appeared in the scientific literature.



#162 VY Canis Majoris

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Posted 23 January 2023 - 04:17 PM

I have compared the IRAS low resolution spectra to the flux in the V-band (taken from the VizieR SED plot), and found that RW Cep's infrared excess is stronger than Rho Cas' by several orders of magnitude. The spectrum instead seems a lot more similar to V915 Sco, one of only very few other K-type hypergiants, although V915 Sco has a weaker 10 micron feature.

#163 VY Canis Majoris

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Posted 24 January 2023 - 04:05 AM

Is there enough to warrant orange or K-type hypergiants as their own type of star? RW Cep and V915 Sco definetly seem a lot more similar to each other than they seem similar to yellow hypergiants, at least in their mid-IR spectrum. With RW Cep still undergoing YHG-like eruptions and having some YHG-like properties it definetly seems like some intermediate to YHGs and RHGs/RSGs.

 

They do seem to be extremely rare, as RW Cep and V915 Sco are likely the only solid examples. Most other stars of this type are normal YHGs that simply pulsate or have eruptions that briefly make them reach this type.



#164 VY Canis Majoris

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Posted 24 January 2023 - 04:45 AM

Pretty sure this has been mentioned before, but there seems to be a bunch of spectra according to VizieR in DAO. There is one from August and one from October both last year. It seems it would be extremely useful to access these and track changes in the continuum or certain spectral features.



#165 robin_astro

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Posted 24 January 2023 - 08:44 AM

Pretty sure this has been mentioned before, but there seems to be a bunch of spectra according to VizieR in DAO. There is one from August and one from October both last year. It seems it would be extremely useful to access these and track changes in the continuum or certain spectral features.

I can see them here

https://vizier.cds.u...e=B/dao/obscore

but the votable files seem to be raw fits images and I have no idea how they could be reduced. Are there reduced versions anywhere?

 

Cheers

Robin



#166 VY Canis Majoris

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Posted 24 January 2023 - 08:55 AM

When I click on the access url it just downloads a file without type called 'datalink', which doesn't even seem to contain the spectrum.



#167 VY Canis Majoris

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Posted 24 January 2023 - 10:19 AM

I've just realized, there are still the spectra in the AAVSO database from 2020 and 2021. I know the continuum shape is unreliable but shouldn't it still be possible to calculate radial velocity?



#168 robin_astro

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Posted 24 January 2023 - 10:46 AM

When I click on the access url it just downloads a file without type called 'datalink', which doesn't even seem to contain the spectrum.

The datalink file is an xml document designed to be used by the Virtual Observatory system but if you look at the xml code, the download link for the fits file is in there. The example file I downloaded (the top one on the list) is a spectrum image which according to the header is ~600A wide ~centred on H alpha. Without the calibration lamp file  there is no way of calibrating it,  though they might still be useful qualitatively.

 

 

I've just realized, there are still the spectra in the AAVSO database from 2020 and 2021. I know the continuum shape is unreliable but shouldn't it still be possible to calculate radial velocity?

They are too low resolution for an accurate RV measurement, (at least 10x lower than the spectra I used) so perhaps +-20km/s at best. In an case comparing  the two spectra there, the wavelength shift between them varies for positive to negative along the spectrum so it seems the wavelength calibration in at least one of them is suspect

 

Cheers

Robin



#169 VY Canis Majoris

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Posted 24 January 2023 - 10:49 AM

Without the calibration lamp file  there is no way of calibrating it,  though they might still be useful qualitatively.

Could this file be retrieved by contacting someone in charge?



#170 robin_astro

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Posted 24 January 2023 - 11:07 AM

Could this file be retrieved by contacting someone in charge?

Even then you would struggle without knowing the intricacies of the spectrograph and  the data reduction pipeline they normally use. (It seems odd they published them in this form) Better to track down the reduced spectra.

 

Cheers

Robin 



#171 VY Canis Majoris

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Posted 24 January 2023 - 04:41 PM

I've tried to get in contact with Roberta Humphreys regarding RW Cephei since she works with hypergiants, however I've never gotten a reponse from the address I sent to (this is not the first time I've contacted). I can't tell if she's really busy or if I somehow have the wrong address. I just feel a really strong sense of urgency as time is running out to collect more detailed data.


Edited by VY Canis Majoris, 24 January 2023 - 04:41 PM.


#172 robin_astro

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Posted 24 January 2023 - 06:22 PM

 I just feel a really strong sense of urgency as time is running out to collect more detailed data.

I suspect interested professional are fully up to speed with this and there is probably plenty going on behind the scenes that we don't know about yet. Interestingly  trying to download the  RW Cep spectra for 2022  from the DAO archive gives an "access denied" message currently.

 

Cheers

Robin



#173 VY Canis Majoris

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Posted 25 January 2023 - 02:36 AM

What could have happened to RW Cephei in 1951? As previously stated both AAVSO and photographic data support this rapid brightening, the former showing its slightly slower recovery. Eruptions only seem to make stars dimmer, and such an eruption was possibly already undergoing at the time.



#174 VY Canis Majoris

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Posted 25 January 2023 - 01:35 PM

So, looking closely at the high resolution spectrum, there seems to be an interesting spectral feature near 7700Å, with a both a strong emission and absorption component. I suspect it would correspond to the potassium K I line, it does not appear to be telluric.

These K I emission lines are apparently rare in stellar spectra but observed in certain hypergiants, RW Cep would be no exception. Maybe I answered my own question but I thought it was worth noting, it does appear to be another indication of circumstellar material.



#175 robin_astro

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Posted 25 January 2023 - 07:37 PM

So, looking closely at the high resolution spectrum, there seems to be an interesting spectral feature near 7700Å, with a both a strong emission and absorption component. I suspect it would correspond to the potassium K I line, it does not appear to be telluric.

These K I emission lines are apparently rare in stellar spectra but observed in certain hypergiants, RW Cep would be no exception. Maybe I answered my own question but I thought it was worth noting, it does appear to be another indication of circumstellar material.

Yes it will be KI 7699A, the same line as I followed in absorption in the eps Aur disc during eclipse

https://britastro.or...5356e4359198628

 

It is a low excitation line so turns up anywhere the temperatures are low.  K7699 is seen in absorption in the interstellar medium for example so given the high extinction it is no surprise to find it in absorption.

 

I don't know the significance of it being in emission but it could be connected with the blue shifted H alpha emission (and various other lines seen in emission currently) which appears to be associated specifically with the dimming, though measurement of K7699 is complicated by the strong  O2 telluric lines in this region which can move across and blend with it depending on the radial and geocentric velocities. The advantage is though if we could dissentangle it from the tellurics we could potentially use the telluric lines to improve the not particularly  accurate wavelength calibration of the high resolution echelle spectra in the BAA database  to give use some radial velocity data. The question is though,  has it changed during the dimming? Unfortunately the ELODIE spectra don't cover that far. 

 

Strong absorption from inter/circumstellar material  is similarly seen in the Na D lines which are covered by the ELODIE spectra. The Na D lines don't show any obvious change however but from the ELODIE spectra, they were already very saturated (from inter/circumstellar material) before the dimming. In the absence of any data prior to the dimming to compare with we will need to follow any changes in the spectrum as it returns to "normal"

 

Cheers

Robin




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