Herschel Observations of Disks around Late-type Stars

The Herschel photometry is extracted using apertures of 5'' and 10'' in radius for 100 and 160 μm, respectively, and a sky annulus from 20'' to 40'' in radius. These radii are the similar to those used by larger Herschel programs for unresolved sources including the DUst around NEarby Stars survey which used radii of 5'' and 8'' at these two wavelengths (Eiroa et al. 2013). The photometric uncertainty is calculated by directly sampling the variation of the background flux away from the target star. Using the same aperture size and aperture correction as for the target star photometry, the background brightness is measured throughout the surrounding sky annulus. In order to account for the correlated noise between neighboring pixels, the uncertainty in the photometry values is estimated by convolving the sky annulus with the photometric aperture size and then estimating its rms. The signal to noise ratio of the detected flux (χ) is then used to determine whether that flux is significantly greater than the stellar photosphere. The significance of an infrared excess is quantified via $\chi =({F}_{\mathrm{obs}}-{F}_{* })/\sqrt{{\sigma }_{\mathrm{obs}}^{2}+{\sigma }_{* }^{2}}$ where F and σ are the flux and errors of the stellar photosphere and observed flux in that bandpass. Table 2 provides the aperture photometry estimated from our Herschel images, χ values and excess flux above the photosphere, F_obs/F_*, in each passband. In Table 2 we also provide estimates of the ratio of the dust luminosity to the stellar luminosity, L_d/L_*, for those stars with both confirmed infrared excesses and non-detections using the formulation given in Beichman et al. (2006). For the stars with non-detections, the L_d/L_* values are upper limits.

The optical and near-infrared photometry for each star is extracted from multiple sources in the literature including TYCHO (Høg et al. 2000), DENIS (Epchtein et al. 1997) and 2MASS (Cutri et al. 2003). The optical stellar fluxes are fitted using a grid of K and M dwarf PHOENIX NextGen spectra (Allard & Hauschildt 1995) which depend on stellar temperature while assuming Solar metallicity. The SED used to model the Herschel photometry is a simple blackbody function given there are at most only two far-infrared data points for our excess stars. The best fit to the complete SED is achieved with the amoeba minimization algorithm by minimizing the χ² value comparing the fit of the model to the observed fluxes. The complete SED of the star and the dust is those components added together and fit to the optical and infrared fluxes. To provide a more physical fit to the data, we utilized the assumption of radiative equilibrium for the dust grains which relates the luminosity of the star (via its temperature and radius) to the temperature and distance of the dust grains. Therefore, the free parameters in our fit are the stellar radius and temperature and dust distance and temperature. The first guesses for the stellar radius and temperature are taken from the Gaia catalog (Gaia Collaboration et al. 2018). The range in which these values are allowed to vary during the fit come from the range of values given in this catalog. The values of the free parameters which produce the best fits to the photometry are provided in Table 3.

A systematic calibration uncertainty (2.75% and 4.15% for PACS100 and PACS160, respectively is also included in the SED fitting (PICC-ME-TN-037 Herschel release note; Balog et al. 2014). The uncertainties on the fitted parameters are estimated by varying each parameter until the reduced chi square value changed by one and holding the remaining free parameters constant at their best fitting values. We also provide a lower limit to the dust mass derived from Jura et al. (1995, Equation (5)) using our estimate of L_d/L_* and assuming spherical grains with a density of 3.5 g cm⁻³, and radius of 1.0 μm. In the case of an object which is not detected at either 100 or 160 μm, a dust luminosity limit is estimated by setting the peak of a blackbody curve to occur at 160 μm, adjusting the dust radius until the combined stellar and dust SED passes through the 160 μm upper limit and adding up the total flux within that blackbody curve.

TYC 9340-437-1 is a β Pic Moving Group member at an age of 23 ± 3 Myr (Torres et al. 2006; Mamajek & Bell 2014; Gaia Collaboration et al. 2018) and a distance of 36.66 ± 0.03 pc (Gaia Collaboration et al. 2018). With fluxes of 88.9 ± 1.7 and 98.6 ± 6.5 mJy at 100 and 160 μm, respectively, this star has significant flux excesses of 51 and 15σ, respectively, at 100 and 160 μm (see Figures 1 and 2). In addition, the disk is spatially resolved at 100 μm (see Figure 1). The FWHM of the dust emission is 23 ± 1'' compared to the Herschel PSF FWHM of 677 at this wavelength (Poglitsch et al. 2010; PACS Observers Manual). An image of the subtraction of the Herschel PSF from the image of TYC 9340-437-1 is also shown in Figure 1. The remaining flux appears to be symmetric at this spatial resolution.

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The convolution of a simple ring model with the PSF model is subtracted from the observed Herschel image and is also shown in Figure 1. The ring model is offset by 14 to match the difference in the nominal and observed target positions (see Table 2). The dust disk is assumed to be radially concentrated into a thin ring with (ΔR/R) of 0.1, a Gaussian radial profile, and an inclination is 7° ± 19° consistent with a face-on disk. Because of its face-on orientation, the position angle of the disk is not well defined. We use an MCMC procedure (emcee; Foreman-Mackey et al. 2013) to find both the best fit parameters and the range of values that are consistent with the observations. The successful fit of a ring model produces a radial spatial extent of the dust of 2.6 ± 02 corresponding to a dust radius of 96 ± 7.4 au. The absence of significant residuals from this model subtraction in combination with the non-detection of any extended emission in Hubble Space Telescope (HST)/NICMOS observations suggests that the orientation of the circumstellar material is consistent with a face-on disk. When we consider the luminosity of the star (0.19 L_☉) and the measured radius of the dust disk, this data lies above the power-law fit but within the scatter of data points in Figure 1 of Matrà et al. (2018) which addresses a potential correlation between these two observables. If we use the best fit line to the Matrà et al. (2018) data, we had expect this disk to have a radius of ∼50 au. Our single new data point does not significantly impact the conclusions from this study that this correlation indicates that there are preferred radii for the formation of planetesimals. The age of this star and the large dust radius could, however, indicate the presence of planetesimal stirring in the absence of a planetary system (Krivov & Booth 2018).

Our SED fit to the photometry for this system results in a dust temperature and radius of 37 ± 10 K and 24 ± 6 au, respectively. If we force the equilibrium dust radius to the 96 au estimated from the resolved image, then we can fit all of the photometry again with the same dust temperature but now with a 1/λ emissivity power law for the dust grains and a grain radius of 1.25 μm. With only two photometry points to characterize the dust, we are hesitant to fit the data with the more complex dust models used in similar studies (e.g., Dodson-Robinson et al. 2016; Morales et al. 2016). To estimate the probability that this detection could be due to a background galaxy, we have consulted Table 2 in Sibthorpe et al. (2013) which provides an integration of the galaxy luminosity function at 100 μm. From this table we estimate that this source has a probability of ∼1.5 × 10⁻⁵ of being a background source. Because this star was pre-selected based on the presence of its mid-IR excess, it is more likely than a randomly chosen star to be contaminated by a far-IR background source. Nevertheless, we conclude, based on the brightness of the observed far-infrared flux and the fact that the emission is spatially resolved, that the likelihood of such contamination is small.

TYC 7443-1102-1 is also a β Pic Moving Group member at a distance of 51.24 ± 0.12 pc and an age of 23 ± 3 Myr (Mamajek & Bell 2014; Gaia Collaboration et al. 2018). The PACS images show a solid detection right at the stellar position. The fluxes are 10.2 ± 1.4 mJy at 100 μm and 17.2 ± 4.5 mJy at 160 μm. This is ∼23 times the photosphere at 100 μm. Fits to the SED of the star produce a dust temperature and radius of 27 ± 4 K and 37 ± 12 au, respectively (see Figure 3). This inner dust radius determined from the SED fit is consistent with the point source seen in the Herschel image which corresponds to an upper limit on the disk radius of <200 au as determined at 100 μm.

TYC-7443-1102-1 was observed with ALMA in band 7 (0.87 mm, 345 GHz) on 2018 August 22 under project 2017.1.01583.S (PI: G. Kennedy). The observations used baselines ranging from 15.1 to 483.9 m from 48 antennae with an average precipitable water vapor of ∼0.76 mm. The total on-source observing duration was 14.62 minutes. J2056-4714 and J1924-2914 were used for pointing, bandpass and flux calibration. J1924-2914 was observed between individual target scans for time-varying atmospheric calibrations. The spectral setup comprised four windows centered on 347.883, 335.883, 333.987 and 345.987 GHz with a width 2 GHz and 128 channels for all but the last window which has a width 1.875 GHz and 3840 channels.

The raw data was calibrated with the provided ALMA pipeline script in CASA version 5.1.2-4 (McMullin et al. 2007). All images were generated with the CLEAN algorithm in CASA. Continuum imaging was carried out using natural weighting and self-calibration was not attempted. This gives a synthesized beam with a position angle of 836 and semimajor and semiminor axes of 043 and 048 respectively. The corresponding image is shown in Figure 4, with a measured rms of 40 μJy beam⁻¹. At the wavelength of observation the star would have a flux density of 4.5 μJy and is not detected. Two distinct sub-mm sources are clearly detected, neither are centered at the Gaia DR2 proper-motion adjusted location of the star. The two sources are 14 and 09 distant from the stellar location and have flux densities of 1.3 mJy and 0.5 mJy, respectively. There is also a smaller peak to the west of the stellar location separated by 017 and with a flux density of 0.1 mJy.

The ALMA absolute pointing accuracy for this observation is ∼30 mas and the error on the Gaia stellar location is submilliarcsecond. Therefore, these mm-wave sources are most likely not associated with the star and constitute background galaxies. For a putative debris disk to be detected with PACS but not with ALMA, the spectral slope of the dust emission would need to have γ ≲ −1, steeper than is seen for well-characterized cases (e.g., Gáspár et al. 2012; MacGregor et al. 2016). Larger surveys (that are less precise) find γ values in the range of −0.5 to −1 (Holland et al. 2017). Thus a scenario where the PACS detection is of a circumstellar disk that is then not detected by ALMA is improbable. Therefore the Herschel excess most likely originated from these contaminating sources and not from a circumstellar disk.

GJ 707 (HIP 89211, HD166348) is a M0V star at a distance of 13.20 ± 0.01 pc (Gaia Collaboration et al. 2018). This star has significant excess emission in just the 100 μm band (see Figure 5). The observed flux at this wavelength is 12.7 ± 2.7 mJy which gives it a 3.3σ excess above the stellar photosphere. This star is also in the Moro-Martín et al. (2015) paper with a flux of 11.3 ± 2.8 mJy. While our flux measurements are consistent, the excess detection at 100 μm is marginal at best. The star is not detected in the publicly available Spectral and Photometric Imaging Receiver (SPIRE) data from the Disk Emission via a Bias Free Reconnaissance in the Infrared and Sub-millimeter (DEBRIS) survey. Our blackbody fit to the Herschel photometry produces a dust temperature and radius of 65 ± 6 K and 6.6 ± 2.1 au, respectively. The hotter temperature and corresponding smaller equilibrium dust radius than those estimated from the other excess stars in our sample is most likely a result of the fit to the single 100 μm photometry point. The Herschel image is unresolved at 100 μm allowing us to place an upper limit of <7'' or ∼92 au on the diameter of its purported disk assuming the above stated stellar distance.

GJ 784 (HIP 99701, HD 191849) is an M0V star at a distance of 6.161 ± 0.002 pc (Gaia Collaboration et al. 2018). With our data analysis, this star has a >3σ excess emission at both 100 and 160 μm (see Figure 5). At 100 μm we estimate a flux of 24.5 ± 1.4 mJy which corresponds to a 10σ detection above the stellar photosphere which is 10.6 mJy at this wavelength. The Herschel DEBRIS survey also included this source in its survey although with a shorter integration time (1686 s versus 721 s). Moro-Martín et al. (2015) report a 100 μm flux of 19.42 ± 2.95 mJy for this star. At this flux the detection is then at 2.95σ significance above the stellar photosphere. Additionally, the peak of the emission seen in our Herschel image is offset with respect to the nominal Herschel position by 2''–3''—within the pointing error for the Herschel telescope (Sánchez-Portal et al. 2014). At the location of this star there are SPIRE data also from Herschel in which there is significant emission at 250, 350, and 500 μm, respectively. When we fit an SED to the Herschel data the resulting inner radius and temperature for the dust are 27 ± 2 K and 25 ± 5 au, respectively (see Figure 3). The Herschel image is unresolved at 100 μm allowing us to place an upper limit of <7'' or ∼43 au on the diameter of the disk assuming the Gaia distance to the star.

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AP Col has been categorized as one of the closest pre-main sequence stars through its lithium equivalent width, photometric variability, astrometry and rotational velocity (Riedel et al. 2011) which is why we included it in our sample. It has an age of ∼40 Myr and is at a distance of 8.4 ± 0.07 pc based on RECONS parallax measurements (Riedel et al. 2011, 2014). Despite its relative young age, this star showed no significant dust emission at either 100 or 160 μm. The photometry values for this source are 3.7 ± 1.6 and 1.0 ± 5 mJy at 100 and 160 μm, respectively, corresponding to a 1.39 and 0.2σ significance at these two wavelengths. We can use the lack of excess emission at these wavelengths to place an upper limit of L_d/L_* < 5 × 10⁻⁵ on any dust emission if we use the 100 μm photometry value and ${L}_{d}/{L}_{* }\lt 6\times {10}^{-6}$ if we use the 160 μm photometry value.