Spectroscopy Section - Recommended Projects

RV Tauri variables

by Ryan Maderak

Spectra will enable studying radial velocity curves and line profiles in order to examine the complex motions within the atmosphere

At maximum light, high resolution (R > 10,000) is required, and high SNR of 80 – 100 per pixel. At minimum light: SNR could be as low as 50. This will require total integration times of 30 - 60 minutes on small scopes.

Wavelength coverage: H-alpha +/– 100 angstroms, erring on the red ward side if spectral range is limited.

Desired cadence: as frequently as possible, at least one spectrum per night.

Mira variables

by Ulisse Munari

Medium-to-low resolution spectrographs (dispersion higher than 2 AA/pixel)

• There are no special requirements to be able to measure the depth of molecular bands (e.g. TiO). A dispersion between 2 and 6 °A/pix will do fine.
• Are there shocks in the pulsating atmosphere (derived from the relative strength of Balmer lines – strength of H-alpha, vs strength of H-beta vs strength of H-gamma)?

High resolution spectrographs (need dispersion 0.2 °A/pix or better)

• Radial velocities (RVs) of Balmer lines, covering the pulsation cycle - Are there long-term variations in the light curves of such objects with time?
• Is there a variation of the shape of the RV curve with the pulsation period?
• Compare RV changes of various types of Mira variables - similarities and differences
• How does the RV curve obtained from measurements at the center of strong molecular bands compare with RV measurements of lines away from molecular bands ? Do we see different amplitudes? Different phases perhaps ?

For both high and low resolution spectra

• How does the profile of an emission line change in shape (multiple peaks, asymmetric shapes, super-imposed sharp absorption components, etc.) along the pulsating cycle? (focus on H-alpha line)

Desired cadence: 1 spectrum per star per week.

Cepheids and RR Lyrae variables

by Róbert Szabó and the TESS RR Lyrae and Cepheid survey team

For Cepheids and RRLs, spectroscopy is particularly useful to determine radial velocity curves of the star, and explore changes of the line profiles along the pulsation cycle. Radial velocity studies are based on as many metallic lines as possible, so echelle spectra (R~10,000) with a broad wavelength range are the best. Also, line profile variations (variations in the shape of the spectral line) can give more detailed information about motions in line-forming regions (and about the geometry of the pulsations, as for those stars the main pulsation is radial). Lower resolution spectra covering the optical range around 4500 - 6500 AA are needed to detect line profile changes during a pulsation cycle.

Cepheids

Either a broad coverage in the optical 4500-6500\AA for precision RVs, or individual lines (such as Halpha, Ca II triplet) that could be expected to show some interesting variability or shock phenomena.

RR Lyrae variables

Broad coverage in the blue/optical 4000-6500\AA, Balmer lines and the D3 Helium line at 5872 AA are of interest. Slightly bluer wavelengths might be better for RR Lyraes, because there are pulsation phases when their spectrum becomes rather featureless.

Integration times should be adjusted based on the pulsation period of the star. A spectrum per 0.05 phase units (or better) is a good coverage for all pulsation periods, especially if the pulsation period is less than a day.

Cepheids with X-ray flashes

Requesting any resolution spectra, preferably 1/day or more frequently. Interesting lines: Balmer (absorption) at different phases of the pulsation period.

Chromospherically active stars

Emission regions: Hydrogen and Ca II H & K lines (chromospheric emission); Absorption features: Ca II H & K lines. Looking for EW changes. Resolution at least 1000. Observe once/week.

Wolf-Rayet (WR) outflows

by Noel Douglas Richardson

Emission: highly ionized He, Si, O, N or C; strong stellar winds with P Cygni absorption profiles; enhanced heavy elements. Lines change with time, as winds evolve and expand. Study long-term evolution of line profiles, and evolution of P-Cygni. Great targets for star analyzers!

Desired cadence: 1 spectrum per night or continuous monitoring.

Semi-detached systems

by Christian Knigge and Stella Kafka

Semi-detached binaries consist of a white dwarf accreting material from a low-mass main sequence or giant donor star. Most of the light observed in those systems is accretion generated (accretion disk, hotspot on the disk, accretion column), and is quite variable in all wavelengths. Depending on the optical state, the spectrum can change from broad emission to broad absorption; depending on the phase of the orbital cycle and the inclination of the system, the line strength may also change. Therefore, both phase-resolved spectra and long-term monitoring provide valuable information on the accretion process and variability of the system.

Snapshot spectra of CVs are abundant in the literature. However there has not been a systematic monitoring of any one system on timescales of years. The AAVSO observers have been meticulous in acquiring photometry on a number of key targets in the north and southern hemisphere, so their photometric behavior is reasonably well characterized. In parallel with photometry, we request similar long-term monitoring of a selected targets. Desired cadence is one spectrum/day or (in the case of phase-resolved targets) one spectrum/0.1 phase units. All resolutions.

Dwarf novae

Spectra requested in quiescence or outburst (and everything in between). Spectra can track changes as the system gets bright/faint, assess differences between quiescence states, compare the rise to/from outburst etc.

Nova-like CVs

Dwarf novae in quiescence or outburst (and everything in between). Spectra can track changes as the system gets bright/faint, assess differences between quiescence states, compare the rise to/from outburst etc.

Microquasars

Microquasars are X-ray binaries which exhibit ultra-relativistic bipolar outflows. In these systems, a stellar mass compact object (either a neutron star or black hole) accretes matter from a supergiant companion star through an accretion disk. Emission lines come from the accretion process (disk, disk hotspot or stream) or the mass-donor star. Spectral lines lines such as H-alpha) and He I line at 6678˚A are usually visible and variable depending on the state of the system. Especially the He I 6678˚A line is valuable for the measurement of reliable radial velocities of the primary component to get a constraint on the orbital parameters of the system.

Spectral resolution of ~1000 or higher are required to measure EW, RV and FWHM variations of those lines, and S/N of more than 20. Exposure times need to be adjusted accordingly.