STIS New Spectrophotometric Standards
Final reduction of GO 12813
September 30 2013

Brian Schmidt, Mike Bessell, Australian National University

Ralph Bohlin, Susana Deustua STSci

Sara R Heap, NASA/Goddard Space Flight Center

Elena Pancino, Giuseppe Altaville, Bologna Observatory (INAF)(Gaia)

Nicolas Regnault, Pierre Astier, l’Institut National de Physique Nucléaire et de Physique de Particules

John Tonry, University of Hawaii

and

Bengt Edvardsson, University of Uppsala

Detlev Koester, University of Kiel

Don Lindler, Sigma Space Corporation

Simon Murphy, University of Heidelberg

Abstract

Uncertainties in the overall calibration of SN to the fundamental standards is currently the largest single source of systematic uncertainty in the Supernova cosmology experiments. Our goal is to tie photometry in both hemispheres to an absolute spectrophotometric system which will serve as the basis of SkyMapper's photometric system, and would be the basis of the calibrations necessary to undertake supernova cosmology experiments to a higher degree of precision. To achieve this we need a network of stars with precisely determined SEDs (to properly account for passband differences) that are on a common photometric zeropoint scale. The highest precision CALSPEC stars have two drawbacks for SkyMapper, they are extremely hot (Teff > 30,000K), and none are in the southern hemisphere. We therefore propose to obtain STIS spectrophotometry between 200nm and 1020nm with an accuracy of better than 1 percent for 13 stars, 6 northern SDSS standards, 6 southern FG metal-deficient dwarfs plus 1 circumpolar solar twin. These stars have colors similar to the bulk of field stars and galaxies and will better sample the survey bandpasses. All these stars have Hipparcos Hp magnitudes accurate to 0.002 mags providing an independent check on the HST photometric zeropoint. They can also be well modelled enabling longer wavelength flux calibrations to be generated. These data would also be used for current SNLS/SDSSII work, DECCAM, and eventually LSST calibration. Furthermore they will be invaluable to calibrate the bright end of the Gaia spectrophotometry. We are also proposing parallel mode WFC3 observations of adjacent fields in six bands to provide additional calibration fields.

Justification

Wide-field CCD surveys underway or planned will provide magnitudes and positions of stars and galaxies with unprecedented internal precision. The first groundbreaking survey in the northern hemisphere was the Sloan Digital Sky Survey (SDSS) which established the now standard very broad u, g, r, i, z passbands and whose excellent photometry continues to be refined through re-calibration (Padmanabhan et al. 2008). Pan-STARRS1 (Tonry et al. 2012), also in the north, is now underway with a g, r, i, z, y survey that extends the photometry deeper and further to the red. The first southern hemisphere survey, SkyMapper (Keller et al. 2007), has commenced. SkyMapper is a 1.3m telescope with a 5.7 sq degree FOV, situated at Siding Spring Observatory in Australia. The imager has 16384x16384 0.5 arcsec pixels and will map the southern sky in 6 bands – u, v, g, r, i, and z. A major driver for all the new surveys is the investigation of Dark Energy.

Our goal with the STIS DDT proposal is to tie photometry in both hemispheres to a precise absolute spectrophotometric system that would serve as the basis of SkyMapper’s photometric system and also allow us to stitch together photometric systems across the entire sky.

Such photometric precision is required for a wide variety of science. These include precision photometric redshifts – used through out cosmology, but specifically for next generation weak lensing surveys. Stellar astronomy also requires precision spectrophotometry for fundamental comparisons with model atmosphere fluxes to substantiate the models’ physical inputs and to use the model fluxes for flux calibration at longer wavelengths.

These stars will also form an essential calibration link for ground based spectrophotometric programs and will help the Gaia satellite tie down their photometric and spectrophotometric systems. It will be the basis of the calibrations necessary to further improve supernova cosmology experiments to a higher degree of precision. Currently, uncertainties in the overall calibration of SN to the fundamental standards are the largest single source of systematic uncertainty in the SN experiments (e.g. Sullivan et al. 2011).

Most ground-based photometric systems are well standardized (Bessell 2005) fainter than 5th mag but ultimately are based on the absolute flux of Vega, measured from the ground in the 1980s and known to only a few percent (Megessier 1995). Most of the uncertainty results from difficulties in measuring the horizontal and vertical atmospheric extinction, and the large temperature difference between the standard lamp and Vega’s photosphere. Although there are plans to improve this situation in the future using modern calibrated Si photodiodes and more sophisticated extinction measurement and modeling, the most reliable photometric calibrations available now are space-based from Hipparcos and HST.

Currently there are two spectrophotometric catalogs from HST – NGSL, and CALSPEC. We had hoped to use stars in the NGSL catalog as our fundamental standards – but due to irreducible remaining uncertainties in the offsets of the stars from the slit centre, there remain uncertainties of at least a few percent in the relative flux as a function of wavelength. The CALSPEC stars, which used a wide slit, do not suffer from this problem, but have other drawbacks that make the current ensemble difficult to use to achieve our goals. These drawbacks are

• there are very few moderate colored stars which can be tied directly to normal stars in the survey - the very blue stars typically require extrapolation from the bulk of the objects we wish to calibrate;
• there are no appropriate southern stars that we can use to tie down that half of the sky;
• there are very few stars that can be tied to an external photometric system with high precision;
• they are based on a single type of model atmosphere, without cross checks to a wider variety of stellar types.

The Hipparcos Hp magnitudes constitute the most precise set of integrated photometric fluxes. There are 72,300 stars brighter than V=10 mag in the Hipparcos Catalogue with median Hp magnitudes given to better than 0.002 mag. These stars can provide very reliable zero-points for all optical and far-red photometric systems. We have demonstrated that the Hipparcos Hp passband is now well characterized (Bessell & Murphy 2012) so we can reliably synthesize the Hp magnitudes from spectrophotometric fluxes, providing an important check/normalisation on the zeropoint of the HST fluxes.

We have selected stars with precise Hp magnitudes, similar to, and including some of the NGSL extremely metal-poor stars, for which we propose to obtain STIS spectrophotometry to better than 1%. The new northern stars are SDSS standards so that we can refine the overall SDSS calibration. One of the southern stars is circumpolar thus providing good calibration for the far south fields as well as a good monitor of extinction. The remaining southern stars were selected at intervals of 4 hours providing good coverage throughout the year and the ability to measure instantaneous extinction through paired observations of two standards, one at high and one at low airmass. The majority of these stars are 9th magnitude metal-deficient weak-line dwarfs and subgiants like the SDSS fundamental standard BD +17 4708, with little or no interstellar reddening. High resolution spectra are also available for all the stars enabling the stellar parameters to be well defined and their energy distributions (SEDs) to be well fitted by model atmospheres. Fitting model fluxes enables them to be used as spectrophotometric and photometric standards out to beyond 25 microns. Combining these new standards with the existing CALSPEC stars provides us with spectra covering a wide range of colors that will enable us to ensure that our system passbands are well defined.

The stars will be excellent to use as standards for ground-based spectrophotometric programs aiming to establish a network of 13th magnitude white dwarf secondary spectrophotometric standards suitable for PanSTARRS (Tonry private communication). Most importantly, the spectra will be invaluable to use as part of the calibration suite of stars for the Gaia BP and RP prism dispersed, slitless spectrophotometry (Pancino et al. 2012a,b).