In 2003 took place
in Oléron a professional/amateur astronomy school organized by
astronomer Jean-Pierre Rozelot, the CNRS – french research
organization – and AUDE association promoting the use of electronic
detectors in amateur astronomy for several decades now. At that
important meeting, amateurs astronomers showed what they were doing
around pro/am collaboration and professional astronomers showed what
their research was about and how amateur could contribute. Several
project kicked off then, such as the Spectro-L yahoo discussion group
and the ARAS web front-end for pro/am spectroscopy.
Coralie Neiner,
astronomer from Paris Meudon observatory, explained to us what Be
stars were and how important it would be to do a long term Halpha
high resolution spectroscopy survey, continuing and expanding the
work Christian Buil had been doing for years. This kicked off the
need for a high resolution spectrograph (the one available at that
time from SBIG had a limited resolving power R<3000) and the
development within AUDE of the Lhires (Littrow High Resolution
Spectrograph).
Later on, a Be
Star Spectra database was created and hosted by Paris Meudon
observatory. A FITS file format was described so amateur software
could produce spectra with the right information in their header –
a long way from the DAT or SPC file format that were used so far.
This BeSS database is now operational for ten years and it was time,
under the lead of Coralie Neiner, to celebrate the 10th anniversary
and to build for new adventures – this was the purpose of the 10th
BeSS symposium that took place in Meudon on october 23rd to 27th,
2017.
It would be
impossible to review everything that was said during this intensive
week but I’ll try to give some summary. The sumposium slides are
available on-line :
A dozen people
gathered for this event, couple professional astronomers (big thanks
to Thomas Rivinius for joining) and some amateurs well involved in
the BeSS project. Around 75 % of the spectra in BeSS were
acquired or archived by one of the person in the room ! Top
« score » amateurs like Christian Buil, Olivier Garde,
Thierry Lemoult and myself were there. It was also international with
Ken Hudson from the USA, but also Jose Ribeiro from Portugal, Hugo
Van den Broeck from Belgium and Arnold de Bruin from the Netherlands.
Jean-Jacques Broussat was reprensenting the beginners and gave us a
nice talk on his experience with BeSS and Be stars spectra in
general. Last but not least, key amateur pilars and founders of the
BeSS & ARASBeAm tools were there with François Cochard and
Valérie Desnoux.
The notes that I am providing here are my own and of course subject to caution as they are my interpretation of what was said there. Basically, any mistakes would be mine! :-)
The first day was
a review of the Be stars and the BeSS database. Coralie Neiner gave a
talk on the Be stars & the Be phenomenon. To summarize, Be
stars are non supergiant B-type (actually O7 to A2) stars that show
or have shown at least once emission in their spectra. Emission is
usually first visible on Halpha, thus the focus on this particular
spectral line in BeSS.
Around 20 %
of the B stars are Be stars and at any given time, around 15 %
of the Be stars are in emission phase. The emission come from a cool
circum stellar disk and hot polar winds. They are fast rotators,
which certainly help for the creation of the disk which is not from
the star formation (ie : it is a deccretion disk, not an
accretion one like for exemple for the young Herbig Ae/Be stars). The
emission profile shape depends on the viewing angle as well as
inhomogeneities in the disk.
Be stars have
variation of several time scale which are visible in photometry and
in spectroscopye (Halpha but not only). They can go from absorption
to emission and back to absorption state within months or years. They
also show short term variations.
One of the main
question regarding Be stars is how the disk is formed, how material
is ejected from the B star. In ten years, several progresses have
been made but questions remains unanswered. The rapid rotation helps.
Some Be stars have a very flat shape and are close to the critical
velocity but still at 90 % of it so it is missing some.
Magnetism
is one aspect of the ejection mechanism. Around 10 % of all hot
stars are magnetic, with a field ‘Bpol’ above 50 Gauss. The
magnetism seem fossil, created while the original material was
collapsing into what became the star. It seems simple structured and
particular strong inside the star. The fossil field remains active
through the (short) life of the hot star. For Be stars, they are then
considered as « Magnetic Be » stars as opposed to
« Classical Be » stars.
The magnetic field
is oblique and does not coincide with the stellar rotational axis.
This leads to a rotational modulation of the longitudinal field
toward the line of sight, photospheric variation if spots are present
on the star, and the UV wind lines.
Wind going through
the magnetic field eject material which get confined on the
equatorial plane of the star. It gives rotational modulation of Xray
emissions, photometric light curve and Halpha emission itself.
The main problem
is that for such hot stars, the fiels is weak, the fast rotation
broaden the line profile making the detection through
spectropolarimetry difficult, there are few spectral lines visible
from thosen hot photospheres and emission lines disturb the
detection !
Omega Orionis is a
peculiar Be stars for which magnetism was detected (Neiner, 2003) but
then not anymore. Material concentrated at the junction of the star
rotation equatorial & magnetic field equatorial planes could be
hidden by the emission but wiped out when an outburst occures
(Neiner, 2012).
Non Radial
Pulsations (NRP) is another explanation. Be stars are
slightly above the main sequence B type stars in the HR diagram,
similar to beta Cep and Slow Pulsating B (SPB) stars, so it was
logical to look for pulsations on those stars. Added to fast
rotation, NRP could bring additional momentum for material ejection.
Mu Centauri was studied and outburst & NRP correlation found
(Rivinius, 1998). A model with quiescence phase followed by a
precursor, the outburst itself and a relaxation phase was described.
COROT satellite
was launched to measure stellar pulsations through very precise
photometry. Exoplanet were also searched by the transit method.
Several Be stars were observed (Neiner, 2012). Compared to models,
observations differed and some additional mixing of the star was
required to match both. In general, the convective cores of Be stars
are 20 % larger and 25 % heavier than the normal B stars.
As a summary, all
Be stars show some level of pulsations, excited by the
kappa-machanism (depending on metallicity) but also stochastic modes
as shown by g-modes visible in the photometry curve study (gravity
g-mode are good to tranport angular mode and those stochastic mode
are visible at time of outburst while we mainly see pressure p-mode
otherwise).
Slow rotators seem
to never go to Be phasis, so the rotation is a key factor. Non Radial
Pulsations seem the key required help, sometimes with magnetism
added. And for several Be stars, binarity does provide complexity.
A quick note to
say that VV Cep is not a Be star as the disk is an accretion disk
from the supergiant main star – this is not the Be phenomenon we
know of.
Also, beta Cep is
a special case with the bet Cep pulsating star and a Be star
companion (linked but further away from interactions).
BeSS database
was launched ten years ago. Professional have lot of archives and
some of them have been uploaded into the database. Amateurs are
mainly observing and putting their spectra in BeSS as they acquire
them.
Echelle spectra
are stored in BeSS with separated orders. Merged spectra are usually
not recommended for line profile analysis so professional astronomers
prefere to work on orders individually.
There are more
than 140000 spectra/orders in BeSS, or around 21000 ‘observations’.
9000 observations were made with R~20000 spectrographs (ie :
Littrow type) and 7000 with R~10000 (ex : echelle)
spectrographs.
91 amateur
astronomers contributed to BeSS (most of them from Europe, 50 %
from France). Six observers represent 50 % of the observations,
16 for 80 % and 28 for 90 %. Most observers have 0,1-0,5m
telescopes.
Two stars have
more than 1000 observations : gamma Cas (this may be too much,
isn’t it?) and delta Sco (due to specific monitoring during
periastron). 38 stars have more than 100 observations. 81 stars with
more than 50 observations.
In high resolution
(R>5000), 92 % of the up to mag 6 stars have a spectrum
within the expected observing period. 88 % up to mag 7 and 74 %
up to mag 8. Considering some targets are not visible as too close to
the Sun and that there are not enough observers in the southern
hemisphere, this is not a bad score !
There were some
discussions regarding the statistics. Halpha monitoring, which is the
primary focus of the long term Be stars spectra monitoring, should be
placed first. A 12 month roling statistics will be added to promote
new observers which need time to catch up with the « all time
top scores ». Also, a new statistics with integration time will
be added in order to promote high SNR and long exposure observations.
Coralie Neiner
presented the BeSS status and some future work. The database
is first a catalog of 2256 classical Be stars, 64 Herbig Ae/Be stars
and 9 other stars.
Herbig Ae/Be were
added to the program several years after it started. They are young
objects (like T Tauri for cooler stars) with an accretion disk. There
are 16 Herbig Ae/Be of magnitude 4-8 which are not very active but 14
of mag 8-9 which would require some more active monitoring. We should
take SNR=30-100 of those stars and look for an absorption on the
right (red) side of the Halpha emission line ; this would be the
signature of an accretion disk beeing formed. If detected, an alert
through Spectro-L discussion group should then be given for frequent
monitoring.
Coralie suggested
to add some new categories into BeSS database and catalog :
For the later one,
the B with forbidden emission lines, a group of few stars with
actually different categories, SNR~30 spectra would be required. Some
are mag 10 or so.
There are around
50000 spectra taken with MUSICOS spectrograph pending validation (a
huge task) as well as spectra from NARVAL and ESO archives waiting to
be uploaded. More validators are required – Olivier Garde, Thierry
Lemoult and myself volunteered to be validator immediately (we had a
training session with Valérie on this) ; Ken Hudson volunteered
to join the team after he gets more experience.
In summary, while
we acquired ten years of data more is required and Halpha is still
the primary goal. Thomas Rivinius highlighted for exemple the work on
omega CMa which was based on 40 years of photometry.
A suggestion was
to define a set of 10-15 bright Be stars for which a very active
monitoring (one spectrum per night, target SNR around 100) would be
required. Without taking too much observing time (bright targets),
this active monitoring could help in better understanding the
outburst process.
SNR information
will certainly be added in BeSS database for each spectrum, and then
displayed on ARASBeAm.
On the second day,
Thomas Rivinius presented his work on Be stars from space
photometry. Based on several space missions (Kepler, COROT, SMEI,
BRITE, MOST…), he focused on several Be stars and analysed large
set of data. For exemple, the SMEI solar telescope scanned the sky
with one strip in 90 minutes, thus for 9 years of data !
He explained that
when the disk, in equilibrum, gets denser, emission from H (~1.6µm)
and K (~2µm) bands come from further away region from the star than
V band (0,5µm). He also highlighted that disks grow and decay both
inside and out. Some significant part of ejected material fall back
onto the star photosphere.
Thomas Rivinius
recommended to monitor the BRITE wiki and observe the same bright
targets in spectroscopy.
He also
recommended to take very high SNR (>300) spectrum of Be stars when
in quiescence in order to get a baseline spectrum.
We then had a
serie of amateur talks. Jose Ribeiro presented the work
on delta Sco 2011 campaign. I presented the status on
V442 And outburst that was
detected in august and closely followed up for more than two months.
An advice to show V/R graph in log scale was given then.
Coralie Neiner and
Thomas Rivinius told us that it is very important to catch Be star
outburst as early as possible (high SNR spectra are best to ensure we
capture faint emission when they begin) and that an almost 24h
monitoring in high resolution is required for the first one to two
weeks. Then observations can be done daily for few weeks then weekly
for couple of months until outburst is done. To achieve this, we
certainly need to expand the BeSS network in the US (Ken Hudson is
running a practical workshop in New Mexico in february ; this
could help) and in the southern hemisphere.
Valérie Desnoux
made a talk on Be stars outburst. Out of the 814
classical Be stars of mag <9 in BeSS, 599 have at least two
spectra. 210 show no change, 135 with some changes (decreasing
emission, V/R variations…) and 254 have at least one outburst. She
extracted a list of 62 « remarquable » stars to watch
for.
Christian Buil
presented his work on other wavelengths
than Halpha. A Littrow (Lhires, L200) spectrograph with 600 gr/mm
grating is very good for near IR, R~4000, work. It can also be used
in the near UV (H & K calcium doublet) to study chromospheric
activites but chromatism is strong ; UVES is a special
spectrograph designed for near UV work.
The eShel can be
modified with a Samyang 135mm F/2 lens, modified lens mechanical
adaptation, UV optical fiber, high temperature (4700K) halogen lamp
used for painting lightning for exemple, a cooled ASI 1600MM camera
or equivalent which shows good performance fo such work. This looks
like a kit to be proposed for eShel users ! Then focus can be
achieved through a large spectral domain including calcium triplet
and Paschen lines in the near IR and H & K in the near UV – all
in one exposure ! This would be perfect with the use of mirror
based telescope such as Newton or GSO 254mm F/8 aluminium telescope.
Near IR spectral
domain seems interesting for Be stars (specially binaries for the
calcium triplet) as well as magnetic Be stars (Paschen lines). The OI
8446 line comes from the same region than the Lyman beta line but is
more transparent making it a good line to study.
H & K near UV
lines are mainly interstellar on Be stars so they are not very
interesting lines. Otherwise, the Balmer discontinuity (with a second
discontinuity from the disk) gives information on the effective
temperature of the star photoshere and the pressure/gravity [log(g)].
Vincent Robert
from the IMCCE presented the NAROO scanning machine project
to scan old photographic plates to get old astrometry and, thanks to
GAIA improved astrometry of reference stars, improve planetary
astrometry for exemple. The new machine beeing built at Paris
observatory will be operated 24h/24h, 7 days a week. It will also be
able to scan spectrographic plates such as the Be stars spectra
recorded at OHP (4400 plates out of a total of 55-80000 plates).
On day three,
Valérie Desnoux howed us how the validation process works. She
developped some specific tools in VisualSpec (anyone can use them!)
to check telluric lines around Halpha and upload last two spectra in
BeSS for comparison.
She also gave some
common mistakes :
-
constant
shift in wavelength of 0.1-0.2A, due to delay with calibration
acquisition
-
in general,
we expect calibration error lower than 0.1A ; this could
(should!) be checked by observer before submitting the spectrum in
BeSS
-
date/time
collision between different spectra, unobservable object
-
wrong
latitude/longitude placing the new observing in the ocean :-)
-
wrong
observing site (OHP typically weeks after OHP workshop!)
-
hot pixels
not well removed, leading to spikes in the spectrum
-
ripples in
the spectrum : can be accepted but warning given to the
observer
-
intensity
saturation, hard to catch but sometimes when comparing with other
spectra
-
continuum
level/shape : can also be accepted with some warning to the
observer to improve
-
bumps in
the continuum that can be real or not (real hard to catch)
-
object
error (ex : filename different from object name) : where
is the mistake ?
-
Signal to
noise ratio too low are accepted but usually a warning and some
advices to improve given to the observer
Valérie proposed
a PDF to add on BeSS web site, giving advice on how to check your own
spectra before submission.
This beeing said,
Valérie mentionned that lot of spectra are ok and that the overall
validation process ensure an excellent data consistency which is a
plus for the database and its reputation among the professional
astronomers community.
Valérie also
showed the monthly reports which desserve to be more
widely communicated. She is spending ot of time to edit those reports
but they highlight the « hot stars » of the moment and
are very useful tools for the observers.
Several feedback
were given on BeSS as well as the ARASBeAm website (and the
associated robot file). While a low resolution program was launched
few years ago (mainly for faint targets and to automatically search
for outbursts ; it didn’t kick off), high resolution Halpha is
our top priority.
The fourth day, we
had several amateur talks. Olivier Garde presented his work on
eta Ori ;
Ken
Hudson showed his installation in New Mexico,
USA ; Jean-Jacques Broussat presented how he got into Be star
spectroscopy and his progress over time ; Hugo Van den Broeck
showed his access to several instruments in Belgium.
Thierry
Lemoult presented his work to automate his observatory and how he
searched (and found) Be
candidates
among
a large group of B stars. Coralie Neiner indicated that you can plot
the Hbeta versus Halpha EW as Be stars should show
off the normal B stars. Also, she encourages us to publish the
results and new Be stars can then be added in BeSS with the
publication referenced.
Thomas Rivinius
gave an excellent talk on spectral
data analysis with lot of tips. He explained how to get
effective temperature and log(g) pressure from Balmer discontinuity.
For Equivalent
Width (EW), the spectrum has to be normalized by taking two
sections of the continuum well outside the line profile and doing a
local linear renormalization. Thomas is actually doing this hundred
times with different random domains ; he then take the median
value of the calculated EW and the RMS scatter as the 1 sigma error
margin. He mentionned that telluric lines do not have a significant
impact so removing them is not critical (this can be tested by taking
a ‘dry’ and a ‘wet’ spectrum and compare the measured EW).
He mentionned
VarTools as a good package for time series data
analysis.
SNR (Signal
to Noise Ratio) is usually easy to calculate on Be stars
where there is lot of continuum. Just flatten a continuum domain and
divide the average value by the RMS. Be careful with the electron
scattering that could cover very large portions of the wings (ex :
chi Oph).
Peak
separation (Huang’s law, Huang 1972) relates to disk size.
This can be done on OI 8446, FeII 5169 or SiII 6347 lines.
V/R ratio
can be measured from zero level, continuum (=1) level, or a profile
modeling. It really doesn’t matter but the method should be
explained/defined in your publications. Zeta Tau shows cyclic (ie :
not periodic) variations with four cycles over 1500 days (Carciofi
2009), indicating a spiral shape of the disk. Pleione is a binary
with 219 days period ; you can easily use BeSS data to fold a
dynamical spectrum and plot two periods for better read out.
You can also
substract spectra taken during quiescence with one
taken during an active phase (exemple with Achenar between 2000 and
2006, or 66 Oph). It does show disk loss. This is why he recommends
to take very high SNR (taking one spectrum in one night for exemple!)
of Be stars during quiescence time.
When Be stars are
in absorption, look for short term changes in the line profile
due to Non Radial Pulsations ; for exemple on 31 Peg, nu Cen,
omega UMA…
Doppler
tomography can be applied for exemple on Hbeta, HeI5876. See
Maintz PhD thesis in 2003 and Jason Grunhut work. Good targets are
phi Per, 59 Cyg, FY CMa, o Pup, HD161306. Remember that in
tomography, the inside of the star is the border of the ‘image’
and that the center is actually the outter space !
Orbital
velocity can show short term cyclicity such as omega Ori.
Look for HeI 4471 line for exemple.
And as already
mentionned, he encouraged us to look for the targets observed by
satellites such as BRITE, K2, TESS and observe in spectroscopy the
bright ones.
Valérie Desnoux
showed some automatic analysis on BeSS data. After ten
years of observations, there are some gold mine waiting to be digged
out. Amateur should work on those and publish the data analysis, for
exemple in IBVS.
Coralie Neiner
then showed some tips for data analysis too :
-
Tlusty is a
good source for O or B type stars models. There are pre-calculated
grids of models (OSTAR2002 and BSTAR2006) that can be downloaded
from the author's website. These grids should be more than enough
for our needs. Start with the file BGvispec_v2.tar available at
http://nova.astro.umd.edu/Tlusty2002/tlusty-frames-BS06.html
-
v.sin(i)
can be measured as first approximation by the Fourrier Transform of
a photospheric line without emission. The first value of the FT
gives the v.sin(i) directly ; spectrum should be scaled in
km/sec of course. Measure it on several spectra to average the
values.
-
Plot
spectra in log(wavelength) to ensure a fix scale in km/s.
The conclusion of
this symposium is that the next one should be in few years only (4-5
years max). We got plenty of « to do » actions out of the
symposium and a motivated team to go out and expand the BeSS
observing community.
Spectroscopy
rocks, Be Stars Spectroscopy even more !
