在巨型黑洞附近发现尘埃与水

利用美国国家航天局（NASA）/欧洲空间局（ESA）/加拿大航天局（CSA）的韦布空间望远镜（James Webb Space Telescope），一个国际天文学家（astronomer）团队发现，在银河系（Milky Way galaxy）中心的超大质量黑洞（supermassive black hole）附近，尘埃和水竟然能够形成并存续。观测结果显示，尽管人马A（Sagittarius A*）周围存在强烈的辐射环境，主序后星（evolved star）IRS 3仍在持续向其周围环境输送新形成的物质。

Using the NASA/ESA/CSA James Webb Space Telescope, an international team of astronomers have discovered that dust and water can form and survive surprisingly close to the supermassive black hole at the centre of our Milky Way galaxy. The observations reveal that the evolved star IRS 3 continues to enrich its surroundings with newly formed material despite the intense radiation environment around Sagittarius A*.

新的观测结果提供了迄今为止关于演化晚期星（highly evolved star）IRS 3最详细的中红外（mid-infrared）图像，该恒星距离银河系中心的超大质量黑洞仅0.55光年（light year）。IRS 3已进入生命末期，即渐近巨星支（asymptotic giant branch）阶段。处于这一生命阶段的恒星巨大、温度较低且光度高，它们通过强大的星风（stellar wind）将气体抛射到太空中。这种抛射出的恒星物质是宇宙尘（cosmic dust）最重要的来源之一，但此前尚不清楚在如此靠近超大质量黑洞的地方，恒星是否还能产生这些物质。

The new observations provide the most detailed mid-infrared view yet of the highly evolved star IRS 3, which is located just 0.55 light-years from Sagittarius A*, the Milky Way galaxy's central supermassive black hole. IRS 3 has reached a stage near the end of its life called the asymptotic giant branch phase. Stars in this phase of life are huge, cool, and luminous, and they shed gas into space with their powerful stellar winds. This cast-off stellar material is one of the most important sources of cosmic dust, but it was not clear if it could be produced by a star so close to a supermassive black hole.

通过利用韦布望远镜的中红外仪器（MIRI）分析该恒星的红外光，研究团队识别出了富氧尘埃的清晰特征，并首次在其周围的厚包层（extended envelope）中探测到了水。研究结果表明，即使在超大质量黑洞附近的严酷条件下，像IRS 3这样的主序后星仍能产生尘埃以及其他对未来恒星和行星（planet）形成至关重要的物质。

By analysing the star’s infrared light with Webb’s MIRI (Mid-Infrared Instrument), the research team identified clear signatures of oxygen-rich dust and, for the first time, detected water in the star’s surrounding envelope. The results show that even under the harsh conditions near a supermassive black hole, evolved stars like IRS 3 can still produce dust and other materials important for the creation of future generations of stars and planets.

“银河系中心是环境最极端的区域之一，因此了解恒星是否能在那里持续丰富其周围环境是一个重要课题，”德国科隆大学的首席作者Florian Peißker表示。“借助韦布望远镜，我们可以直接观察恒星在这些条件下的表现，并发现尘埃的产生依然具有惊人的韧性。”

“Galactic centres are among the most extreme environments, so understanding whether stars can continue enriching their surroundings there is an important question,” said lead author Florian Peißker of the University of Cologne in Germany. “With Webb, we can directly observe how stars behave under these conditions and see that dust production remains remarkably resilient.”

IRS 3是银河系中心最明亮的中红外源（infrared source）之一，长期以来因其巨大的尘埃包层而引人注目。先前的研究认为该恒星可能是富碳的，但新的观测结果呈现了不同的图景。韦布望远镜的数据揭示了两个与硅酸盐尘埃相关的强红外特征，这种尘埃由硅和氧组成。这些特征表明IRS 3是一颗富氧的演化晚期星，它正处于生命末期并向太空抛射物质。

IRS 3 is one of the brightest mid-infrared sources in the galactic centre and has long stood out because of its enormous dusty envelope. Previous studies suggested the star could be carbon-rich, but the new observations paint a different picture. The Webb data revealed two strong infrared signatures associated with silicate dust, which is composed of silicon and oxygen. These features identify IRS 3 as an oxygen-rich evolved star, which is nearing the end of its life and shedding material into space.

“这一发现归功于韦布望远镜功能强大的红外仪器，”ESA的研究合著者兼MICONIC项目首席研究员Macarena Garcia Marin说道。“这是首次为该恒星收集到连续的中红外光谱（infrared spectrum），使我们能够探测到硅酸盐尘埃的特征，并揭示该恒星真实的化学身份。”

“This discovery was possible because of Webb’s highly capable infrared instruments,” said Macarena Garcia Marin of ESA, a co-author of the study and PI of the MICONIC programme. “This is the first time a continuous mid-infrared spectrum has been collected for this star, allowing us to detect the features from the silicate dust and uncover the star’s true chemical identity.”

通过将韦布望远镜对该恒星光谱的观测结果与光线穿过不同包层模型时的模拟相结合，研究团队得以重建该恒星包层的结构。他们的结果显示，尘埃呈层状壳层分布，从恒星向外延伸约10 000天文单位（astronomical unit），温度从靠近恒星处的约1200开尔文下降到外围区域的约100开尔文。观测结果还揭示了IRS 3包层内存在水的证据：这是该天体首次被明确探测到水。

By combining Webb’s observations of the star’s spectrum with simulations of how its light would move through different models of the surrounding envelope, the team were able to reconstruct the structure of the star’s envelope. Their results indicate a layered, shell-like distribution of dust extending roughly 10 000 astronomical units from the star, with temperatures falling from approximately 1200 Kelvin close to the star to around 100 Kelvin in the outer regions. The observations also revealed evidence for water within the envelope of IRS 3: the first clear detection of its kind for this object.

“水的探测尤其令人兴奋，因为它表明分子物质可以在强辐射主导的环境中存续，”Macarena说。“这告诉我们，即使在靠近超大质量黑洞的地方，恒星也能持续向其周围环境贡献物质。”

“The detection of water is especially exciting because it shows that molecular material can survive in an environment dominated by intense radiation,” said Macarena. “This tells us that even close to a supermassive black hole, stars can continue contributing material back into their surroundings.”

根据观测和恒星建模，研究人员估计IRS 3的质量约为太阳的六倍，年龄约为72百万年（million years）。该恒星似乎正在经历剧烈的质量损失（mass loss），将物质喷射到太空中，并形成了韦布望远镜所观测到的厚包层。

From the observations and stellar modelling, the researchers estimate that IRS 3 has a mass of approximately six times that of the Sun and is around 72 million years old. The star appears to be undergoing intense mass loss, ejecting material into space and creating the extended envelope seen by Webb.

这些结果表明，演化晚期星可能在向银河系中心输送尘埃方面持续发挥重要作用——这些区域此前被认为对这些过程极其不利。

These results suggest that evolved stars may continue playing an important role in supplying dust to galactic centres — regions that were previously thought to be especially hostile to these processes.

这些观测数据于2025年获得，是利用韦布望远镜的MIRI仪器，作为“附近标志性星系中心中红外表征”（MICONIC）保证时间观测项目的一部分。

The observations were obtained in 2025 as part of the Mid-Infrared Characterisation of Nearby Iconic galaxy Centres (MICONIC) Guaranteed Time Observations programme using Webb’s MIRI instrument.