银河系内历史性超新星的光回声Light Echoes from Historical Galactic Supernovae

田文武

Light echoes have previously been detected only from a handful of objects, e.g. Supernovae 1987A and 1993J, Nova Persei 1901 etc.

Reference paper: Krause, O. et al. 2008, Nature, 456, 617Tycho Brahe's 1572 supernova as a standard type Ia explosion revealed from its light echo spectrum

科技日报 刘霞 天文学家捕获超新星光“回声”

图片说明：布拉赫超新星残余 . 来源： Max Planck Institute for AstronomyMax Planck Institute for Astronomy

德国、日本和荷兰的科学家通过研究遥远的星际尘埃云团的光线，确定了“死”于 436 年前的布伦赫超新星是 type1a 超新星的一种，这一发现为暗能量研究提供了重要线索。论文刊登在 12 月 4 日的《自然）杂志上。

A. 两类超新星1. 核心坍缩型超新星

(SNII、SNIb,、SNIc)

2. 吸积白矮星的热核爆炸型超新星(SNIa)

两类超新星的主要特征超新星类型 Ia II极大光度 3 x 109 L⊙ 3 x 108 L⊙

光谱 无氢光谱线 ; 重元素光谱线很多 ; 后期 Fe线最强

最强的是氢光谱线

前身星 双星系统中的白矮星 大质量恒星

爆发原因 伴星物质被致密白矮星吸积而流入白矮星

大质量恒星的铁核心坍缩

爆发物理机制 吸积白矮星 C/O 核心的热核大爆炸 转化为铁族元素

从新生中子星表面向外行进的反弹激波 : 中微子压强

残骸 无致密残骸 中子星超新星遗迹内的核产物 主要是铁 各种元素都有

Tycho1572 的发现和研究它的意义1572年丹麦天文学家第谷在仙后座附近发现了一颗很可能是超新星的爆发。他当时认为这颗星距离地球非常遥远。这个发现彻底颠覆了 2000多年的天文学理论—亚里士多德的“地球是宇宙的中心”理论 . 根据历史记录 Tycho 1572很可能是 Ia类型的超新星 , 即由双星系统中的白矮星发生热核爆炸而成的；所以 Ia类型的超新星的研究为白矮星的诞生和灭亡提供了证据 ; Ia超新星达到最大发光强度时的绝对等级几乎是一定的，这一特征使热核爆炸型超新星被广泛用作测定到远方星系距离的标准光源 .

白矮星为什么爆炸和爆炸的方式还没有很合理的解释 . 一种理论 : 白矮星从其伴星那里不断地获取新物质，当其重量超过 1.4个太阳质量时导致热核爆炸。另一种理论则认为爆发是由于一颗大质量的恒星吞另一颗白矮星而导致的。

Detections and Geometry of SN Light Echoes

Light echoes may be produced from variable and cataclysmic stars, if the light pulse is sufficiently luminous and the dust is sufficiently dense to scatter the pulse (Sugerman, B. 2003, ApJ. 126, 1939)

1: Visible light echoes have been detected around historical Supernovae (SNe) due to reflecting light pulse by dust in the ISM. Sample 1: tycho SN 1572.

2: Infrared (IR) light echoes from ancient SNe have also been detected due to the echoed re-radiation of thermal energy as the light pulse from a SN passes through its surrounding ISM and heats the dust. Sample 2: Cassiopeia A.

Assuming a dust column behind the SN, the grey region in each parabola defines the dust hit and heated by light from the initial outburst at a specific time, t1<t2<t3, since the SN outburst. This dust may either reflect light flash to produce the light echoes or thermally re-radiate light to produce the IR light echoes. Given a distance to a SN, scattering angle and r can be determined well. z=1/2(h2/ct – ct)

Ellipsoid

Assuming d >>r

Paraboloid

Geometry of light echo propagation

Light echo arclets associated with Tycho 1572. First-epoch image is from 2006 Oct. 20 (top left) and the second-epoch image from 2007 Dec. 13 (top right) with a size of 325’× 250’. Bottom panels show difference images between the two top images where white represents the later (2007) image and black the earlier image. Saturated bright stars are masked gray. The left bottom panel is repeated in the right with the motion vectors plotted. Red represents a straight line fit to the arclet, yellow the apparent motion of the arclet, and blue means the reverse vector direction. The surface brightness in the brighter arclets is roughly 24 mag/aresec^2. The widths of the echoes are typically 10” across.

From The Astrophysical Journal Letters, 2008, 681(2):L81–L84.For permission to reuse, contact journalpermissions@press.uchicago.edu.

Using 4 m telescopeat Kitt Peak National Obs.Filter’s Centralwavelength: 594.5 nm,FWHM: 212.0 nm Echo arclets have apparentmotions of 20”-40” /yr

Images at 24 μm of moving features (bottom panels) about 45 pc in projected distance from Cassiopeia A in the northern region (boxed subregion). Cas A is the big bright blob on the lower left side of the top image. The images were obtained on 2005 Feb. 3 (epoch 3), 2005 Sep. 1 (4), 2006 Feb. 14 (5), and 2006 Oct. 2 (6). Nearly all the bright features in this region are IR light echoes, and they are especially evident in the circles (morphology changed).

From The Astrophysical Journal, 2008, 678(1):287–296.For permission to reuse, contact journalpermissions@press.uchicago.edu.

Using Multiband Imaging

Photometer for Spizer.

Observed in scaning mode

for large map (120’x120’),

in photometry model for small

maps (20’x20’).

Basic information: about 400 yrs old; a circular-like radio shell with the brightest to the NE; faint optical filaments /knots to the NW, NE and E; a X-ray shell with brighter to the NE; HI absorption gives its distance of 2–5 kpc; optical proper motion and shock velocity gives ~2.4 kpc. Likely Type 1a SN (maybe Type 1b/IIL).

Radio image Infrared imageX-ray image

Tycho 1572: a historic Galactic SN

1: Type Ia SNe are thermonuclear explosions of white dwarf stars in close binary systems. They are cosmological distance indicators and have led to the discovery of the accelerated expansion of the Universe.

2: The proximity of the SN 1572 remnant has allowed the possible identification of the binary companion, and provides a unique opportunity to test theories of the explosion mechanism and the nature of the progenitor.

3: Detecting optical spectra of the SN near maximum brightness confirms that SN 1572 is a normal SNe Ia.

The spectra type of Tycho 1572

Figure 1. Optical images of the SN 1572 light echo. Panels a) and b) show R-band images of the same area (120”x120”, using telescopes of 3.5 m at Calar Alto, Spain and 8.2 m on Mauna Kea, Hawaii, respectively). The position of the brightness peak in the first epoch is marked for reference. The rectangle shown in a) indicates the location of a previous light echo detection. The vector towards SN 1572 is indicated (arrow). The seeing for panels a) and b) was 1.5” and 0.9”, FWHM, respectively. Integration times were 20 and 12 min. A long-slit spectrum of the brightness peak of the echo structure in b) was extracted, covering the wavelength range of 3800 to 9200 A with a spectral resolution of 24 A.

Fig. 2. Spectrum of SN 1572

has been corrected for the

colour dependence of the

scattering process for a

scattering angle of ~ 84° and

de-reddened for a foreground

extinction of AV =4.2 mag.

The comparison spectra has

been obtained from the time

average of light curve weighted spectra at days (relative to maximum brightness) -5, -4, -2, +2, +4, +10, +11, +24, +50, +76 for SN 1994D and days -9, -4, +1, +9, +18, +40 for SN 2001el and from days -20 through 70 for the normal SN Ia template1.

Comparison of SN 1572 with SNe Ia of different luminosities. Spectral templates of subluminous, normal and overluminous type Ia SNe are shown in comparison with the spectrum of SN 1572. The comparison spectra have been derived as the time average of a spectral series over days 0-90 after explosion and scaled to the spectrum of SN 1572. Specific features typical for the three subtypes are indicated.Figure 3.

结论 1.以前已经捕捉到了光的回声，但来自于 Tycho 1572 的爆炸 是在银河系看到的爆炸，找到并研究了它的光回声并通过与 过去河外发生的超新星爆发的光谱进行详细比对，证明第谷 超新星爆发属于标准的热核爆炸型超新星爆发 (SN 1a)。 2.Type Ia超新星是极亮的爆炸，在可观测的宇宙中大多数地方 都能看到，由于它们在显示宇宙的加速膨胀的测定工作中所 起的作用，它们已成为有广泛科学兴趣的天体。 最近几年揭 示 Type 1a SN比我们想象中的要远，因此一种神秘的名为暗 能量的力量正在推动星系相互远离，因为 Tycho 1572 在银河 系中 , 距离我们很近 (2.4千秒差距) ，研究它的残留物能够帮 助天文学家更好地理解 type1a超新星和暗能量本身 .

3.天文学家怀疑 Tycho 超新星是 Type 1a中的一类，但一直无法 证实，这个观察让一切盖棺论定

The Cassiopeia A Supernova Was of Type IIb Oliver Krause, et al. Science, 2008, 320, 1195.

1.Cassiopeia A is one of the youngest supernova remnant (~340 yr) known in the Milky Way and a unique laboratory for supernova physics. 2. The optical spectrum of the Cas A supernova near maximum brightness is obtained from observations of a scattered light echo more than three centuries after the direct light of the explosion swept past Earth. 3. Cas A was a type IIb supernova and originated from the collapse of the helium core of a red supergiant that had lost most of its hydrogen envelope before exploding.

Fig. 1 IR images of the echo region (A to D: 24-µm images of 2.5’ by 5’, with observing epoch labeled on each panel). The bright IR light echo is visible at the center of (C). Other echoes ~60’ north of this feature are indicated by strong morphological changes in the time series; for example the compact echo in (A), which disappeared later, in contrast to the smooth and unchanged interstellar cirrus emission. The white rectangle in (C) denotes the size of the optical images of the bright echo region.

Fig. 2. Optical images of the echo region. (A to C) show R-band images of the same area of 33” by 33”. The white rectangle shown in all three panels denotes the 2” by 2.1” extraction aperture of the Cas A SN spectrum. Seeing was 1.6”, 0.7”, and 1.3”, FWHM, for A, B, and C, respectively. The contours in B denote 24-µm emission from Fig. 1C in steps of 25 MJy/sr starting at 55 MJy/sr. The size of the MIPS 24-µm beam is indicated in the lower right (circle). The contours in C display 24-µm emission from Fig.1D with the same levels plotted in B.

Fig. 3. Spectrum of Cas A supernova and SN 1993J. Spectral features are labelled with their rest wavelength given in Å. The spectrum was extracted from the aperture shown in Fig. 2 and binned to 11.2 Å per pixel.

More Outcomes from Detected Light Echoes1. Study the three-dimensional structure of the ISM surrounding young SNRs (on a scale of 1 parsec or sub-parsec). See Kim et al. 2008, ApJ, 678, 287.2. Estimate ages or distances of young SNRs and pulsars associated. See Rest, A. et al. 2005, Nature, 438, 1132. More…3. Be distance indicators to galaxy clusters. Braun & Milgrom, 1989, ApJ, 337, 644.4. It may be in connection with Gamma-ray bursts. See Reichart , D.E. 2001, ApJ, 554, 463. 5. Probe the baron density of the universe with high-z source. See scholomitskii, 1997, Ap&SS, 252, 177.

我的兴趣和工作• 天文学上距离是最难确定的数据之一 . 此前人

们根据一些观测结果推测第谷超新星残骸距地球的距离介于 7000 光年至 1.6 万光年之间，而本项研究成果使得我们可以推定第谷超新星距离。

• HI+CO 测距；消光测距 (X-ray and 21 cm –- 氢的柱密度 ---Av) ．

我们的方法：ＨＩ＋１３ＣＯ• 一：他们采取一种直接环绕源区选取背景的方法来构

建 HI 的吸收谱线。这种方式虽然导致背景中包括部分源区辐射，但他们的理论分析证明这种方式不会损失光深，反而因背景直接与源区相接，可以最大程度地避免假 HI 吸收谱线的产生，因而给出最可靠的天体距离（范围）。• 二，他们同时分析同一天体方向的 13CO 发射谱线。

CO 和 HI 谱线揭示星系内介质完全不同的气体成分（ 13CO 谱线揭示光学薄的分子云）。 比较同一天体方向的 13CO 发射谱线和 HI 吸收谱线，可以找到与产生（主要） HI 吸收谱线相对应的氢分子云，因此可以进一步限定天体的运动学距离（因没有产生相应 HI 吸收谱线的氢分子云应在天体的后面）。

An example to build absorption spectra to Kes 75 (a TeV source)

1420 MHz map from VLA D-array observations.

21 cm HI map from VLA+GBT observations with contours (28 40 60 100 k) overlaid

Ton (v)-Toff(v)=(Ts-Tbg)(1-e^{-tau})

solid-line box ---> Ton(v) Annulus background Toff(v)

• HI+CO13 spectra show that the highest absorption velocity~95 km/s, absence of absorption at ~102 km/s, the tangent point velocity ~110 km/s. So d ~ 5.1 to 7.5 kpc

• Rotation

curve of

Milky Way

stays flat

with

distance

Calculated radial velocity as a function of distance along a line of sight in the Galactic plane at l = 29.7 degreeFormula: Vr= Ro sin l (V(R)/R – Vo/Ro). V(R):circular orbit speed of a star; R:Galactocentric radius; Vr:radial velocity

２００７来相关联的成果• 1. Tian W.W., Leahy D.A., Haverkorn M., Jiang B.

2008 ， ApJ Letters ， 679 ， L85 。• 2. Tian W.W., Li Z., Leahy D.A., Wang Q.D. 2007, ApJ

Letters, 625, L25. • 3. Tian W.W., Leahy D.A. 2008 ， MNRAS Letters ， 391,

54

• 4. Leahy D.A., Tian W.W., Wang Q.D. 2008, AJ, 136, 1477 .

• 5. Tian W.W., Leahy D.A. 2008, ApJ, 677, 292.• 6. Leahy D.A., Tian W.W. 2008, A&A Letters, 480, L25. • 7. Leahy D.A., Tian W.W. 2008, AJ, 135, 167. • 8. Tian W.W., Leahy D.A. ， Wang Q.D. 2007, A&A,474,

541. • 200 ８年审稿 A ｐＪ两篇 , MNRAS 一篇 , 天文学进展一篇

y

我们一起作更多工作．．．• １０多颗有良好历史记录的河内超新星

遗迹．• ２ . １６米望远镜．• ＬＡＭＯＳＴ谱观测．• 红外…