Young Vega and Altair Analogs: Rotationally-Enhanced Activity in HD 169142 and HD 135344

Debris disks associated with rapidly rotating A stars, such as Vega, are among the nearest candidate planetary systems. These stars differ from Solar analogs in having stronger UV radiation fields with implications for the evolution of planetary atmospheres and the survival of molecular gas in the disk to the epoch of giant planet formation. Both moderately rotating A stars ( PsA, Kalas et al. 2005) and rapidly rotating A stars (Vega, Marsh et al. 2006) have structure in their disks suggesting that planets have formed.

By combining inclination data derived from either scattered light imaging of the disk or from millimeter studies with v sin i data from the literature, we have identified several systems which appear to be young Vega analogs in that the stars are rapidly rotating, and the disks are viewed close to face-on. Here we concentrate on two of these stars: HD 169142 and HD 135344.

Star HD 169142 HD 135344 Spectral Type A5Ve F4Ve v sin i (km/s) 55 69 Inclination 13 ± 2° 11 ± 2° Veq (km/s) 240 360 Age (Myr) 6 ± 4 8 ± 4 Distance(pc) 140 140 Disk outer radius

1.6” 220 AU

1.05” 145 AU

Accretion rate (solar masses yr-1)

<7x10-10 <5x10-9

B-V 0.31 0.48 E(B-V) <0.08 0.013

Both HD 169142 and its co-moving binary companion were detected in a 10ks Chandra ACIS-S image (left, Grady et al. 2007a; is up and East to the left in the image). The Herbig Ae star can be fit with a single temperature thin thermal model (the APEC code) kT=0.16 keV, NH=3.6x1021 cm-2 model (histogram, with data-model residuals below, right), resulting in Lx~1029 erg s-1 for d=140 pc. The pulse height spectrum more closely resembles 51 Eri (F0V, t=12 Myr, Feigelson et al. 2006) than either young, non-accreting Herbig Ae stars such as HD 163296 (Swartz et al. 2005) or accreting Herbig Ae stars.

HD 135344’s circumstellar disk has also been detected in scattered light with NICMOS PSF-subtracted coronagraphy at both 1.1 m and, in a re-analysis of 1.6 m archival data, to an angular distance of ~ 1.05” (~ 150 AU) from the star.

C. A. Grady1, G. Schneider2, K. Hamaguchi3, M. Sitko4, W. Carpenter5, K. Collins6, G. Williger6, B. Woodgate7, R. Petre7, J. Nuth, III7, D. Hines8, T. Henning9, A. Quirrenbach9, F. Ménard10, D. Wilner11 1Eureka Scientific and GSFC, 2U. of Arizona, 3USRA and GSFC, 4Space Sciences Institute and U. Cincinnati, 5U. Cincinnati, 6U. Louisville, 7NASA's GSFC, 8Space Sciences Institute, 9MPIA, Germany, 10Laboratoire d'Astrophysique de Grenoble, France, 11CfA.

I. The Stars

III. HD 169142 in X-rays

HD 135344 HST/NICMOS F110W

HD 135344 HST/NICMOS F160W

Light scattered from the HD 169142 circumstellar disk has been imaged with NICMOS 1.1 m PSF-subtracted coronagraphy to an angular distance of 1.4” from the centrally occulted star (~ 200 AU in pole-on projection), spatially coincident with millimeter detection of the disk to 1.6” (Raman et al. 2006). The radial surface brightness (SB) of the disk declines as r-3, indicative of a geometrically-flat disk that is directly illuminated by the star (Whitney & Hartmann 1992). Modeling of the IR spectrum suggests an inner cavity ~ 24 AU in radius with material in the inner disk which does not appreciably shadow the outer disk.

HD169142

T Tauri companions

At r > 0.62”, the disk has an very steep radial SB profile, r-9.7, suggesting that the outer disk is both geometrically flat and shadowed rather than directly illuminated. As noted by Dominik et al. (2005), the disk is very vertically extended at the dust sublimation radius, suggesting that this star continues to accrete some material from its disk.

II. The DisksHD 169142 HST/NICMOS F110W

The accreting Herbig Ae star HD 163296 has a harder X-ray spectrum than seen in HD 169142 (Swartz et al. 2005).

HD 163296

The ROSAT HRI detected an x-ray source (1RXH J151548.5 -370917) 1.06” from HD 135344 with a count rate of 0.009 +/-0.0015 c/s. This is equivalent to the ROSAT HRI count rate for HD 163296, scaled to 140 pc, suggesting a similar Lx=1.5x1029 erg s-1.

This luminosity is consistent either with low-level accretion plus some coronal activity from HD 135344, or with the entire Lx

originating on a low-mass companion. The NICMOS coronagraphic data exclude a companion with spectral type earlier than M2 at r > 0.85” from the Herbig star.

a) HD 169142 appears later in spectral type in the UV than its optical type of A5Ve would indicate. b, c) The star also lacks the strong emission and wind absorption lines seen in accreting Herbig Ae stars (HD 104237, orange). However, there is enhanced FUV emission compared to non-accreting systems (Altair, blue), and with stronger emission in some chromospheric transitions such as C II. No HH knots are seen in ACS F122M imagery, above the background surface brightness limit of 1.2x10-13 erg cm-2 s-1 arcsec-2 at 1”< r < 12”.

HD 135344 has a FUV spectrum (black) characterized by a faint continuum at longer wavelength (<f> = 5x10-15 erg cm-2 s-1 Å-1) and emission in C III 977, 1176, and O VI 1032, 1038. The O VI profile is narrow compared to jet-driving Herbig Ae stars such as MWC 480 (blue), and is similar to other low v sin i older Herbig stars such as HD 139614 (red). The absence of the blue-shifted jet component in O VI suggests that the current accretion rate onto HD 135344 is ~3-5x10-9 solar masses yr-1, scaling down from MWC 480 (10-8 solar masses yr-1). The stellar photosphere contributes no more than 5x10-16 erg cm-2 s-1 Å-1 (e.g. HD 181327, F5V, veq = 33 km/s). Collectively, these data indicate that the bulk of the emission is due to stellar activity, as expected for an F star.

IV. HD 135344 in X-rays

V. UV and FUV ActivityHD 169142 HD 135344

VI. Conclusions •By combining inclination data derived from circumstellar disk studies with projected rotational velocites, we can now separate the rapid rotators from more typical A-F stars.

•HD 169142 and HD 135344 exhibit enhanced x-ray and emission-line activity over that observed for more typically rotating young A-F stars, including systems like 51 Eri (Feigelson et al. 2006). For HD 135344, the majority of this emission is from stellar activity: at most 20% of the FUV line emission is from accretion. For HD 169142, our Chandra data suggest that stellar activity dominates the X-ray data.

•Neither star has the accretion rate inferred from model fits to the mid-IR spectral energy distribution (D’Allessio et al. 2005). Emission-line based measures of accretion (Garcia-Lopez et al. 2006) may prove more robust.

•When combined with FUV data, the X-ray data for older late-A to F Herbig stars suggest a gradual change over to activity dominated by magnetic activity rather than accretion, with the process complete by 12 Myr ( Pic Moving Group).

•Herbig Ae stars known to drive jets are shown with green stars. Stars with open symbols are debris disks. Both HD 135344 and HD 169142 lie at the upper end of the observed x-ray activity for Pic Moving Group (BPMG) F stars, and well above BPMG brown dwarfs or A stars.

Multi-wavelength Data Rules! This study included data obtained with the NASA/ESA Hubble Space Telescope as part of HST-GO-10177, HST-GO-10764, data from the Chandra X-ray Observatory under program P7200493, FUV data obtained with the Far Ultraviolet Spectrographic Explorer under program E510, and archival spectra obtained with the International Ultraviolet Explorer. Data analysis facilities were provided by the Exoplanets and Stellar Astrophysics Lab at GSFC, the Steward Observatory of the University of Arizona, and the Department of Physics and Astrophysics at the Johns Hopkins University.

a) b)

c) d)

OVIC III