[论文解读] Ruling out unresolved binaries in five transitional disks VLT/NACO: deep 2.12 and 1.75 μm narrow-band imaging

S. Vicente, B. Merín+9 more|HAL (Le Centre pour la Communication Scientifique Directe)|Jul 15, 2011

Astrophysics and Star Formation Studies参考文献 94被引用 11

一句话总结

本研究利用甚大望远镜/NACO的深度近红外窄带成像，排除了五颗演化过渡盘在中心恒星0.1角秒范围内的未resolved双星伴星，内腔/间隙区域的对比度极限为2–5 mag，外区可达9.3 mag。结果表明，恒星和亚恒星伴星并非大多数系统内腔形成的主因，支持其他机制如行星形成或尘埃颗粒增长。

ABSTRACT

Aims. We aim at detecting the presence of companions inside the inner hole/gap region of a sample of five well known transitional disks using spatially-resolved imaging in the near-IR with the VLT/NACO/S13 camera, which probes projected distances from the primary of typically 0.1 to 7 arcsec. The sample includes the stars DoAr 21, HD 135344B (SAO 206462), HR 4796A, T Cha, and TW Hya, spanning ages of less than 1 to 10 Myr, spectral types of A0 to K7, and hole/gap outer radii of 4 to 100 AU. Methods. In order to enhance the contrast and to avoid saturation at the core of the point-spread function (PSF), we use narrow-band filters at 1.75 and 2.12 μm. The "locally optimized combination of images" (LOCI) algorithm is applied for an optimal speckle noise removal and PSF subtraction, providing an increase of 0.5-1.5 mag in contrast over the classic method. Results. With the proviso that we could have missed companions owing to unfavorable projections, the VLT/NACO observations rule out the presence of unresolved companions down to an inner radius of about 0".1 from the primary in all five transitional disks and with a detection limit of 2 to 5 mag in contrast. In the disk outer regions the detection limits typically reach 8 to 9 mag in contrast and 4.7 mag for T Cha. Hence, the NACO images resolve part of the inner hole/gap region of all disks with the exception of TW Hya, for which the inner hole is only 4 AU. The 5σ sensitivity profiles, together with a selected evolutionary model, allow to discard stellar companions within the inner hole/gap region of T Cha, and down to the substellar regime for HD 135344B and HR 4796A. DoAr 21 is the only object from the sample of five disks for which the NACO images are sensitive enough for a detection of objects less massive than \sim 13 MJup that is, potential giant planets or low-mass brown dwarfs at radii larger than \sim 76 AU (0".63).

研究动机与目标

• 检验未resolved双星伴星是否可解释演化过渡盘中观测到的内层消光空洞。
• 探究近距离双星引起的潮汐截断是否为原行星盘中清除尘埃间隙的可行机制。
• 约束五颗研究较充分的演化过渡盘内腔/间隙区域潜在伴星的质量与轨道半径。
• 通过在高对比度成像中应用先进的散斑噪声抑制技术，提升对低质量伴星的探测灵敏度。
• 在排除双星伴星作为成因后，评估其他机制（如行星形成、颗粒生长或光致蒸发）在空洞形成中的可行性。

提出的方法

• 利用甚大望远镜上的NACO仪器，在1.75 μm和2.12 μm波段获取深度窄带成像，以提升对比度并避免点扩散函数（PSF）核心区域的饱和。
• 应用局部优化图像组合（LOCI）算法以抑制散斑噪声并改善PSF减去，相比传统方法对比度提升0.5–1.5 mag。
• 使用在相似条件下观测的PSF定标星，建模并减去恒星PSF，从而实现对主星附近微弱伴星的探测。
• 基于观测到的对比度极限计算5σ灵敏度曲线，并结合演化模型，估算在不同距离处伴星的最小可探测质量。
• 评估不同盘区的探测极限，区分内腔/间隙区域（0.1–1.5角秒）与外盘区域（大于1角秒）。
• 引入更新的恒星参数（如TW Hya的更年轻年龄和M2.5光谱型），以优化对亚恒星与行星质量天体的质量灵敏度估计。

Figure 1: LOCI optimization grid for a full width at half-maximum (FWHM) of 6 pixels overlaid on the 1.75 $\mu$ m PSF-subtracted image of HD 135344B (09-05-2007). The frame is 512 $\times$ 512 data pixels (13.27 mas/pixel) and the diameter of the inner circle is $\sim$ 52 pixels. The FWHM input used

实验结果

研究问题

• RQ1 未resolved双星伴星能否解释演化过渡盘中观测到的内层空洞或间隙？
• RQ2 在每颗演化过渡盘的内腔区域内，可被排除的伴星最小质量是多少？
• RQ3 VLT/NACO成像的灵敏度如何限制盘区中恒星、棕矮星或行星质量伴星的存在？
• RQ4 成像结果在多大程度上排除了双星潮汐截断作为盘空洞起源的可能性？
• RQ5 更新的恒星参数（如年龄和光谱型）如何影响盘中低质量伴星的可探测性？

主要发现

• VLT/NACO观测在所有五颗演化过渡盘中，均排除了主星0.1角秒范围内的未resolved伴星，内区对比度极限为2.2–4.9 mag。
• 在外盘区域（大于1角秒），对比度极限达到7.7–9.3 mag，其中T Cha的对比度极限为4.7 mag。
• 观测结果排除了在距离超过~5.5 AU（0.1角秒）处质量超过0.11 M⊙的恒星伴星，以及在7 AU（0.13角秒）以外的棕矮星质量伴星。
• 在~13 AU（0.24角秒）以外，行星质量伴星（低至~7 M_Jup）被排除，灵敏度在87 AU起延伸至8 mag对比度。
• 对于DoAr 21，观测对半径大于~76 AU（0.63角秒）的伴星具有灵敏度，可探测质量小于~13 M_Jup的天体，从而能够发现潜在的巨行星或低质量棕矮星。
• 结果支持其他机制（如巨行星形成、高效颗粒生长或光致蒸发）作为空洞形成的主因，尤其在DoAr 21和HR 4796A中，双星伴星被排除为主要成因。

Figure 2: High-contrast residual images after PSF removal with the LOCI algorithm for both filters NB_1.75 and NB_2.12. Each panel is a $3\aas@@fstack{\prime\prime}3\times 3\aas@@fstack{\prime\prime}3$ sub-image centered on the target star. The bottom set of panels is a zoom-in of the inner $1^{\pri