An Adaptive Optics Survey of Stellar Variability at the Galactic Center

We present a $\approx 11.5$ year adaptive optics (AO) study of stellar variability and search for eclipsing binaries in the central $\sim 0.4$ pc ($\sim 10''$) of the Milky Way nuclear star cluster. We measure the photometry of 563 stars using the Keck II NIRC2 imager ($K'$-band, $\lambda_0 = 2.124 \text{ } \mu \text{m}$). We achieve a photometric uncertainty floor of $\Delta m_{K'} \sim 0.03$ ($\approx 3\%$), comparable to the highest precision achieved in other AO studies. Approximately half of our sample ($50 \pm 2 \%$) shows variability. $52 \pm 5\%$ of known early-type young stars and $43 \pm 4 \%$ of known late-type giants are variable. These variability fractions are higher than those of other young, massive star populations or late-type giants in globular clusters, and can be largely explained by two factors. First, our experiment time baseline is sensitive to long-term intrinsic stellar variability. Second, the proper motion of stars behind spatial inhomogeneities in the foreground extinction screen can lead to variability. We recover the two known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We constrain the Galactic center eclipsing binary fraction of known early-type stars to be at least $2.4 \pm 1.7\%$. We find no evidence of an eclipsing binary among the young S-stars nor among the young stellar disk members. These results are consistent with the local OB eclipsing binary fraction. We identify a new periodic variable, S2-36, with a 39.43 day period. Further observations are necessary to determine the nature of this source.Comment: 69 pages, 28 figures, 12 tables. Accepted for publication in The Astrophysical Journa

Long-lived space observatories for astronomy and astrophysics

NASA's plan to build and launch a fleet of long-lived space observatories that include the Hubble Space Telescope (HST), the Gamma Ray Observatory (GRO), the Advanced X Ray Astrophysics Observatory (AXAF), and the Space Infrared Telescope Facility (SIRTF) are discussed. These facilities are expected to have a profound impact on the sciences of astronomy and astrophysics. The long-lived observatories will provide new insights about astronomical and astrophysical problems that range from the presence of planets orbiting nearby stars to the large-scale distribution and evolution of matter in the universe. An important concern to NASA and the scientific community is the operation and maintenance cost of the four observatories described above. The HST cost about $1.3 billion (1984 dollars) to build and is estimated to require$160 million (1986 dollars) a year to operate and maintain. If HST is operated for 20 years, the accumulated costs will be considerably more than those required for its construction. Therefore, it is essential to plan carefully for observatory operations and maintenance before a long-lived facility is constructed. The primary goal of this report is to help NASA develop guidelines for the operations and management of these future observatories so as to achieve the best possible scientific results for the resources available. Eight recommendations are given

Probing Interstellar Grain Growth through Polarimetry in the Taurus Cloud Complex

The optical and near-infrared (OIR) polarization of starlight is typically understood to arise from the dichroicextinction of that light by dust grains whose axes are aligned with respect to a local magnetic field. The sizedistribution of the aligned grain population can be constrained by measurements of the wavelength dependence of the polarization. The leading physical model for producing the alignment is that of radiative alignment torques (RAT), which predicts that the most efficiently aligned grains are those with sizes larger than the wavelengths of light composing the local radiation field. Therefore, for a given grain size distribution, the wavelength at which the polarization reaches a maximum (max) should correlate with the characteristic reddening along the line of sight between the dust grains and the illumination source. A correlation between max and reddening has been previously established for extinctions up to AV 4 mag. We extend the study of this relationship to a larger sample of stars in the Taurus cloud complex, including extinctions AV > 10 mag. We confirm the earlier results for AV </p

Training of Instrumentalists and Development of New Technologies on SOFIA

This white paper is submitted to the Astronomy and Astrophysics 2010 Decadal Survey (Astro2010)1 Committee on the State of the Profession to emphasize the potential of the Stratospheric Observatory for Infrared Astronomy (SOFIA) to contribute to the training of instrumentalists and observers, and to related technology developments. This potential goes beyond the primary mission of SOFIA, which is to carry out unique, high priority astronomical research. SOFIA is a Boeing 747SP aircraft with a 2.5 meter telescope. It will enable astronomical observations anywhere, any time, and at most wavelengths between 0.3 microns and 1.6 mm not accessible from ground-based observatories. These attributes, accruing from the mobility and flight altitude of SOFIA, guarantee a wealth of scientific return. Its instrument teams (nine in the first generation) and guest investigators will do suborbital astronomy in a shirt-sleeve environment. The project will invest $10M per year in science instrument development over a lifetime of 20 years. This, frequent flight opportunities, and operation that enables rapid changes of science instruments and hands-on in-flight access to the instruments, assure a unique and extensive potential - both for training young instrumentalists and for encouraging and deploying nascent technologies. Novel instruments covering optical, infrared, and submillimeter bands can be developed for and tested on SOFIA by their developers (including apprentices) for their own observations and for those of guest observers, to validate technologies and maximize observational effectiveness.Comment: 10 pages, no figures, White Paper for Astro 2010 Survey Committee on State of the Professio

FLITECAM: early commissioning results

We present a status report and early commissioning results for FLITECAM, the 1-5 micron imager and spectrometer for SOFIA (the Stratospheric Observatory for Infrared Astronomy). In February 2014 we completed six flights with FLITECAM mounted in the FLIPO configuration, a co-mounting of FLITECAM and HIPO (High-speed Imaging Photometer for Occultations; PI Edward W. Dunham, Lowell Observatory). During these flights, the FLITECAM modes from ~1-4 μm were characterized. Since observatory verification flights in 2011, several improvements have been made to the FLITECAM system, including the elimination of a light leak in the FLITECAM filter wheel enclosure, and updates to the observing software. We discuss both the improvements to the FLITECAM system and the results from the commissioning flights, including updated sensitivity measurements. Finally, we discuss the utility of FLITECAM in the FLIPO configuration for targeting exoplanet transits

Gemini Imaging of Mid-IR Emission from the Nuclear Region of Centaurus A

We present high spatial resolution mid-IR images of the nuclear region of NGC 5128 (Centaurus A). Images were obtained at 8.8 micron, N-band (10.4 micron), and 18.3 micron using the mid-IR imager/spectrometer T-ReCS on Gemini South. These images show a bright unresolved core surrounded by low-level extended emission. We place an upper limit to the size of the unresolved nucleus of 3.2 pc (0.19") at 8.8 micron and 3.5 pc (0.21") at 18.3 micron at the level of the FWHM. The most likely source of nuclear mid-IR emission is from a dusty torus and possibly dusty narrow line region with some contribution from synchrotron emission associated with the jet as well as relatively minor starburst activity. Clumpy tori models are presented which predict the mid-IR size of this torus to be no larger than 0.05" (0.85pc). Surrounding the nucleus is extensive low-level mid-IR emission. Previously observed by ISO and Spitzer, this paper presents to date the highest spatial resolution mid-IR images of this extended near nuclear structure. Much of the emission is coincident with Pa-alpha sources seen by HST implying emission from star forming areas, however evidence for jet induced star formation, synchrotron emission from the jet, a nuclear bar/ring, and an extended dusty narrow emission line region is also discussed.Comment: 22 pages, 6 figures, accepted by Ap

Unprecedented Near-infrared Brightness and Variability of Sgr A*

The electromagnetic counterpart to the Galactic center supermassive black hole, Sgr A*, has been observed in the near-infrared for over 20 yr and is known to be highly variable. We report new Keck Telescope observations showing that Sgr A* reached much brighter flux levels in 2019 than ever measured at near-infrared wavelengths. In the K' band, Sgr A* reached flux levels of ~6 mJy, twice the level of the previously observed peak flux from >13,000 measurements over 130 nights with the Very Large Telescope and Keck Telescopes. We also observe a factor of 75 change in flux over a 2 hr time span with no obvious color changes between 1.6 and 2.1 μm. The distribution of flux variations observed this year is also significantly different than the historical distribution. Using the most comprehensive statistical model published, the probability of a single night exhibiting peak flux levels observed this year, given historical Keck observations, is less than 0.3%. The probability of observing flux levels that are similar to all four nights of data in 2019 is less than 0.05%. This increase in brightness and variability may indicate a period of heightened activity from Sgr A* or a change in its accretion state. It may also indicate that the current model is not sufficient to model Sgr A* at high flux levels and should be updated. Potential physical origins of Sgr A*'s unprecedented brightness may be from changes in the accretion flow as a result of the star S0-2's closest passage to the black hole in 2018, or from a delayed reaction to the approach of the dusty object G2 in 2014. Additional multi-wavelength observations will be necessary to both monitor Sgr A* for potential state changes and to constrain the physical processes responsible for its current variability

An Adaptive Optics Survey of Stellar Variability at the Galactic Center

We present an ≈11.5 yr adaptive optics (AO) study of stellar variability and search for eclipsing binaries in the central ~0.4 pc (~10'') of the Milky Way nuclear star cluster. We measure the photometry of 563 stars using the Keck II NIRC2 imager (K'-band, λ_0 = 2.124 μm). We achieve a photometric uncertainty floor of Δm_K' ~ 0.03 (≈3%), comparable to the highest precision achieved in other AO studies. Approximately half of our sample (50% ± 2%) shows variability: 52% ± 5% of known early-type young stars and 43% ± 4% of known late-type giants are variable. These variability fractions are higher than those of other young, massive star populations or late-type giants in globular clusters, and can be largely explained by two factors. First, our experiment time baseline is sensitive to long-term intrinsic stellar variability. Second, the proper motion of stars behind spatial inhomogeneities in the foreground extinction screen can lead to variability. We recover the two known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We constrain the Galactic center eclipsing binary fraction of known early-type stars to be at least 2.4% ± 1.7%. We find no evidence of an eclipsing binary among the young S-stars nor among the young stellar disk members. These results are consistent with the local OB eclipsing binary fraction. We identify a new periodic variable, S2-36, with a 39.43 days period. Further observations are necessary to determine the nature of this source