RS Oph : the contribution of Shelyak spectrographs

By Olivier Garde

Introduction

A Nova is a “new star” that appears in the sky. In fact, this star existed long before it was suddenly accessible to our instruments (even in a few rare cases to our eyes), but its brightness increases very suddenly because of its outburst. In general, this explosion does not destroy the star, and it may even be that these explosions are recurring – the general mechanism is now fairly well known.

A Nova is always an “event” in astronomy: if it can be observed very regularly in spectroscopy as soon as it is discovered, we observe a very rapid evolution – on the scale of a few minutes, then of a few days – of its spectral signature. This evolution teaches us a lot about the physics and kinematics of the explosion; it is a magnificent laboratory for astrophysicists.

A Nova is also the privileged opportunity for the community of amateur astronomers to make a very useful contribution to Science. Often these objects are accessible to small instruments. And above all, by their reactivity and their geographical distribution in the world, amateurs can do what professionals often do not know (anymore?): Observe very quickly and very regularly. However, it is clear that the more quickly we observe a nova, the more precise information we will derive from it: over time, the explosion is diluted.

On August 8, 2021, the recurrent nova RS Oph was observed in outburst at magnitude V = 5. The amateur community immediately mobilized to realize many spectra at various resolutions with various spectrographs (with resolving powers R = 600 to R = 30,000).

Between August 8th, 2021 (date of the announcement of the outburst of RS Oph) and August 23th, 2021 (date of the 1st publication with amateur data), 142 spectra were produced and deposited in the ARAS spectral database. More than 3/4 were performed with spectrographs from the range offered by Shelyak Instruments: Alpy 600, LISA, LHIRES III, UVEX, eShel and a Whoppshel.

75% of observers are equipped with Shelyak spectrographs:

  • 4 eshel
  • 1 Whoppshel
  • 6 Alpy
  • 6 LHIRES III
  • 2 LISA
  • 2 UVEX
 

Note that in the “Others” category which represents 25% of observers, 2 users produced spectra with a UVEX and a Sol’Ex in 3D printing

Users of Shelyak spectrographs produced 78% of the spectra with the following distribution:

  • 46 eShel spectra
  • 27 Alpy spectra
  • 26 Lhires III spectra
  • 5 LISA spectra
  • 5 UVEX spectra
  • 3 Whoppshel spectra

The greatest number of spectra was carried out with an eShel spectrograph at R = 11,000 (46 spectra), then we find in second almost equal spectra LHIRES III (26 spectra) and Alpy 600 (27 spectra). The users of UVEX and LISA spectrographs produced 10 spectra and the Whoppshel at R = 30,000, 3 spectra.

In the “other” category, 6 spectra were produced with a Sol’Ex and 5 with a UVEX in 3D printing.

Examples of spectra

Here are some examples of spectra made with the various spectrographs Shelyak Instruments.

Alpy 600

With a resolution of R = 600, the Alpy covers the entire visible range, or even part of the near UV. Taking into account the magnitude of RS Oph at the time of the acquisitions, only a 10s exposure is necessary while taking care not to saturate the very intense H Alpha line.

 

Spectrum taken with an Alpy 600 by Vincent Lecocq 🇫🇷

Spectrum taken with an Alpy 600 by Pascal Le Dû 🇫🇷

LISA

Like the Alpy 600, the LISA covers the entire visible spectral range with a higher resolution at R = 1.000

 

Spectrum taken with a LISA by David Boyd 🇬🇧
The spectrum is calibrated in absolute flux in erg/cm2/s/Å

Spectrum taken with a LISA by Kevin Bazan and Jean-Loup Lemaire 🇫🇷

LHIRES III

Many spectra have been made with the LHIRES III.The strong point of this spectrograph is to be able to adapt its resolution according to the magnitude of the target. During this measurement campaign on RS Oph, users did not hesitate to change gratings. Many spectra have been performed at wavelengths other than H Alpha

Spectrum taken with a LHIRES III by Paolo Berardi 🇮🇹
Near IR spectrum between 8150 and 8900 Å with 1200gr/mm grating

Spectrum taken with a LHIRES III by Erik Bryssinck 🇧🇪
On the H Alpha line with a 2400gr/mm grating.

 

UVEX

The new spectrograph of the Shelyak range, the UVEX was used by 2 observers in the USA and Australia with 2 different gratings (300 gr/mm and 600 gr/mm)

Spectrum taken with an UVEX 300 gr/mm by Sean Curry 🇺🇸
in the near UV part.

Spectrum taken with an UVEX 600 gr/mm by Peter Velez 🇦🇺
In the near UV part.

eShel

The eShel spectrograph produced the most spectra on this RS Oph campaign with a resolution R = 11,000 (46 spectra). Thanks to its “echelle” optical design, it covers the entire visible spectral range.

Spectrum taken with an eShel by Olivier Thizy 🇫🇷
Detail of a spectrum in the H Alpha region

 

Spectrum taken with an eShel by Stéphane Charbonnel 🇫🇷
Between H beta and H Alpha

 

Whoppshel

The most resolving spectrograph in the Shelyak range, the Whoppshel (R=30.000) could be used here on RS Oph during the 2 nights where the target’s magnitude was less than mag. V = 5. The spectrograph was used with a 400mm telescope, but the Whoppshel gives its full potential with 1m diameter telescopes and allows lower magnitudes to be achieved.

Spectre réalisé avec un Whoppshel par Olivier Garde 🇫🇷
Détail d’une partie du spectre dans la région de H Alpha

Spectre réalisé avec un Whoppshel par Olivier Garde 🇫🇷
Détail d’une partie du spectre dans la région de H Beta

Sol’Ex-> Star’Ex

We can achieve very good spectra with a high resolution 3D printed spectrograph: The Sol’Ex here used in its Star’Ex version

Sol’EX/Star’Ex spectrum taken by Christian Buil 🇫🇷

Here the Sol’Ex is in its original configuration: 10µm slit, 2400 gr/mm grating. It makes it possible to reach a resolution of R = 30.083. The optics used here is a simple 65mm diameter telescope.

Conclusion

RS Oph’s outburst has mobilized many amateur astronomers around the world, producing spectra with various instruments and at different spectral resolutions.

It’s still far too early to learn all the scientific lessons from this nova – the observations will continue for several more weeks, even several months (you can of course contribute yourself). Science needs time to move forward! In fact, even if the general principles of a nova are understood today, each nova has its own specificities, and it is precisely the difference between the particular case observed and the cases previously known that allow us to develop our knowledge. In this case, the first feedback from the scientific community about RS Oph confirms the interest of this particular star.

For example, Steve Shore, Astrophysicist, had a wide temporal and spectral coverage to study the phenomenon, and he has already written three “Astronomical Telegrams” (ATel) to date:

ATel #14868 from 23 August 2021
ATel #14881 from 28 August 2021
ATel #14883 from 29 August 2021

A great example of teamwork!