193 research outputs found
The dependence of the structure of planet-opened gaps in protoplanetary disks on radiative cooling
Planets can excite density waves and open annular gas gaps in protoplanetary
disks. The depth of gaps is influenced by the evolving angular momentum carried
by density waves. While the impact of radiative cooling on the evolution of
density waves has been studied, a quantitative correlation to connect gap depth
with the cooling timescale is lacking. To address this gap in knowledge, we
employ the grid-based code Athena++ to simulate disk-planet interactions,
treating cooling as a thermal relaxation process. We establish quantitative
dependences of steady-state gap depth (Eq. 36) and width (Eq. 41) on planetary
mass, Shakura-Sunyaev viscosity, disk scale height, and thermal relaxation
timescale . We confirm previous results that gap opening is the
weakest when thermal relaxation timescale is comparable to local dynamical
timescale. Significant variations in gap depth, up to an order of magnitude,
are found with different . In terms of width, a gap is at its narrowest
around , approximately to narrower compared to the
isothermal case. When , it can be wider, and higher
viscosity enhances this effect. We derive possible masses of the gas
gap-opening planets in AS 209, HD 163296, MWC 480, and HL Tau, accounting for
the uncertainties in local thermal relaxation timescale.Comment: 19 pages, 16 figures, 4 tables, accepted for publication in Ap
Multi-granularity Item-based Contrastive Recommendation
Contrastive learning (CL) has shown its power in recommendation. However,
most CL-based recommendation models build their CL tasks merely focusing on the
user's aspects, ignoring the rich diverse information in items. In this work,
we propose a novel Multi-granularity item-based contrastive learning (MicRec)
framework for the matching stage (i.e., candidate generation) in
recommendation, which systematically introduces multi-aspect item-related
information to representation learning with CL. Specifically, we build three
item-based CL tasks as a set of plug-and-play auxiliary objectives to capture
item correlations in feature, semantic and session levels. The feature-level
item CL aims to learn the fine-grained feature-level item correlations via
items and their augmentations. The semantic-level item CL focuses on the
coarse-grained semantic correlations between semantically related items. The
session-level item CL highlights the global behavioral correlations of items
from users' sequential behaviors in all sessions. In experiments, we conduct
both offline and online evaluations on real-world datasets, verifying the
effectiveness and universality of three proposed CL tasks. Currently, MicRec
has been deployed on a real-world recommender system, affecting millions of
users. The source code will be released in the future.Comment: 17 pages, under revie
Improve Transformer Pre-Training with Decoupled Directional Relative Position Encoding and Representation Differentiations
In this work, we revisit the Transformer-based pre-trained language models
and identify two problems that may limit the expressiveness of the model.
Firstly, existing relative position encoding models (e.g., T5 and DEBERTA)
confuse two heterogeneous information: relative distance and direction. It may
make the model unable to capture the associative semantics of the same
direction or the same distance, which in turn affects the performance of
downstream tasks. Secondly, we notice the pre-trained BERT with Mask Language
Modeling (MLM) pre-training objective outputs similar token representations and
attention weights of different heads, which may impose difficulties in
capturing discriminative semantic representations. Motivated by the above
investigation, we propose two novel techniques to improve pre-trained language
models: Decoupled Directional Relative Position (DDRP) encoding and MTH
pre-training objective. DDRP decouples the relative distance features and the
directional features in classical relative position encoding for better
position information understanding. MTH designs two novel auxiliary losses
besides MLM to enlarge the dissimilarities between (a) last hidden states of
different tokens, and (b) attention weights of different heads, alleviating
homogenization and anisotropic problem in representation learning for better
optimization. Extensive experiments and ablation studies on GLUE benchmark
demonstrate the effectiveness of our proposed methods
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