First Results from High Angular Resolution ALMA Observations Toward the HL Tau Region (1503.02649v3)

Abstract: We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0.075 arcseconds (10 AU) to 0.025 arcseconds (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analogue HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46.72pm0.05 degrees) and position angle (+138.02pm0.07 degrees). We obtain a high-fidelity image of the 1.0 mm spectral index ($\alpha$), which ranges from $\alpha\sim2.0$ in the optically-thick central peak and two brightest rings, increasing to 2.3-3.0 in the dark rings. The dark rings are not devoid of emission, we estimate a grain emissivity index of 0.8 for the innermost dark ring and lower for subsequent dark rings, consistent with some degree of grain growth and evolution. Additional clues that the rings arise from planet formation include an increase in their central offsets with radius and the presence of numerous orbital resonances. At a resolution of 35 AU, we resolve the molecular component of the disk in HCO+ (1-0) which exhibits a pattern over LSR velocities from 2-12 km/s consistent with Keplerian motion around a ~1.3 solar mass star, although complicated by absorption at low blue-shifted velocities. We also serendipitously detect and resolve the nearby protostars XZ Tau (A/B) and LkHa358 at 2.9 mm.

• The paper demonstrates that sub-0.1 arcsecond ALMA images resolve HL Tau’s disk into bright and dark rings, indicating active planet formation.
• It employs elliptical modeling to determine disk properties, yielding an inclination of approximately 46.72° and a position angle of 138.02°.
• Spectral observations of HCO+ reveal Keplerian rotation around a 1.3 M⊙ star, with absorption features suggesting complex gas-disk interactions.

ALMA Observations of HL Tau: High Angular Resolution Insights into Protoplanetary Disk Structures

The paper "First Results from High Angular Resolution ALMA Observations Toward the HL Tau Region" presents significant findings derived from the Atacama Large Millimeter/submillimeter Array (ALMA) data on the HL Tau region. The angular resolutions achieved at various wavelengths, notably 2.9, 1.3, and 0.87 mm, provide unprecedented insights into the structures of circumstellar disks, particularly the disk surrounding the young star HL Tau. The authors report an intricate pattern of bright and dark rings, suggesting interference effects that could be indicative of ongoing planet formation processes.

Key Observations and Methods

The paper resolves the HL Tau disk with angular resolutions finer than 0.1 arcseconds, corresponding to spatial scales of approximately 10 AU down to 3.5 AU. The researchers have conducted a thorough analysis of the disk orientation parameters, finding an inclination of approximately 46.72° and a position angle of 138.02°. These values are determined by fitting elliptical models to the ring structures observed in the ALMA continuum images.

The high-resolution images reveal multiple rings with varying brightness and spectral index values, leading to conclusions about the optical depth and grain growth processes within the disk. The spectral index (α) value in different areas ranges from 2.0 in optically thick regions to 2.3-3.0 in the dark rings, consistent with a scenario where these have experienced some degree of grain growth or evolution. Such variations strongly support the presence of azimuthal structures potentially due to young planets forming and influencing the disk material.

Molecular Gas Dynamics

ALMA's capabilities extend beyond continuum imaging to spectral line observations. The paper describes the detection and kinematic mapping of HCO$^+$ (1-0) moving at velocities consistent with Keplerian rotation around a central mass, estimated to be approximately 1.3 M$_{\odot}$. However, the analysis is complicated by absorption effects at specific velocity ranges, indicating complex gas-disk interactions, possibly due to outflows affecting the absorption profiles.

Implications and Future Research

The detection of such detailed features provides tangible evidence for early stages of planet formation, corroborated by the presence of mean motion resonances intra- and inter-ring dynamics. The inclination and eccentricity variations suggest the presence of gravitational interactions possibly with forming planets, offering a glimpse into the planetary system dynamics in their nascent stages.

These results have critical implications for our understanding of how planets and planetary systems form and evolve. The paper sets a new benchmark for observing star and planet formation, emphasizing the transformative impact of high-resolution ALMA observations in astrophysics. The findings also hint at the potential to explore the physical processes governing protoplanetary disk evolution with an eye towards determining how typical these processes might be in other stellar environments.

Future developments in observational technologies and techniques, alongside improvements in theoretical modeling, will be required to leverage these observations fully. In particular, these advancements will be crucial to differentiate between different grain growth and disk instability scenarios and to refine models of nascent planet formation processes. As these methodologies evolve, they will enable a deeper understanding of the diversity in planetary system architectures, enriching our comprehension of both exoplanetary systems and our Solar System's origins.